On the topic of gold economy: getting 3CDs and 2 markets out (even before campus districts) is an essential move. Not for gold though, but rather for the culture.
Personally, I tend to get 4. Most often the order is one in my first and second city (even though the campus will be placed first), and two-three cities where CD is placed first (and campus later).
What happens later and how many CDs you need is heavily dependent on the map and difficulty. I will post some general guidance that does not depend on any particular CIV:
Squeezing gold out of AI:
Deity/Immortal:
Here the AI will be rocking with ridiculous amounts of gold. If you manage to finish warmongering at around T100 or so, before you meet other CIVs you can count on being able to supplement your income with often ridiculous trade deals. If you keep your cities small (and maybe have a Colosseum) amenities will not be a problem even with a large number of cities. AI peace deals will also be very lucrative and can provide you with a super push. So keep trading and re-trading luxuries & strategics, relics and great works (if any) - and safely plan to get at least ~100GPT in the end game / probably more.
Emperor and below:
Here, the AI will rarely have much gold (or will be borderline bankrupt) for most of the mid-late game. They will have some gold early on, so key lump trades will provide you with a needed push (e.g. gold for builders to chop settlers, battering rams etc). Otherwise you cannot count on the AI much, if at all. In any case, keep trading and selling everything.
Rely on Commercial City States:
Normally you will have at least one commercial city state, usually two, and if there's three thats amazing. Make sure you cultivate commercial city states, finish quests (many of which will come naturally) and get 3 then 6 envoys - so you're looking at: (4+8 x #CD GPT) per city state. The more commercial city states you find, the less own CDs you need.
Where the traders come from:
Obviously, from CDs you build. But how many you need to self-build also depends on how many traders you can get otherwise:
Traders from AI:
For whatever reason, AI very rarely prioritises CD. I dont think I ever captured one in my initial expansion. In the late game expansion, sometimes the AI surprises you with one. On average, expect no CDs built for you.
Harbours however are a different story: of course it depends on the map and other civs, but a quick expansion on another continent will usually yield at least 2-3 harbours if you're lucky 4-5. In any case, In any case these extra harbours should be treated as a supplement (that can make you a bit more relaxed in the late game).
Traders otherwise:
*You will get 2 traders from the merchant republic. As a rule of thumb, in my quick games this usually brings the number of available trade-routes to 7-9 (with 5 already filled) which means its the stage where you're still developing/chopping your empire.
*Traders from Great Merchants. In any case, expect at least one. On lower difficulties - you may easily get 3 GMs that expand your trade rout capacity. On higher, usually not. This is a great balance, as it somewhat compensates for the fact that AI does not have good gold for trades.
And now the most important piece of advice:
Get your traders out immediately.
This means, except maybe for the first trader, all other traders must be chopped! In my experience, and if you follow the general SV strategies, one trader will be immediately finished in exactly one chop. Even in a city with one population.
I believe that making traders slowly in all your cities is the biggest mistake possible. There are always places around the map where you can harvest a trader. So plan for it - 'count' the woods/stone in your cities. Decide where you will be chopping traders (it may be one-two cities, or it may be scattered) and keep your builders available and on hold.
I'll give an example: any city where you will not be pursuing a third district is good enough. Or, if you examine the Gorgo save (linked) Once I captured England, I got a 2 charge builder. Used the builder to chop another builder and then virtually chopped every wood/deer england had (six) to put out 6 traders (3 I believe to fill the the harbour slots England left, and 3 to finish my own as they became available). This also means I totally ignored these cities. Never produced anything there except repairing the monuments (didn't have the tech to fix the harbour as I generally don't research Astrology and Celestial Navigation up until the very end (and even then, only if I want to harvest Crabs - and even then, I do it through pre-selecting these and finishing them in one turn with any of the lump science GS (e.g. Charles Darwin etc).
If you're stuck, buy a builder in a weak city. Lets say you pay 500 gold for a 5 charge builder. And net 5 traders at turn 90 or so (made up number). These traders will be running 5 x 50 turns = 250 trade/moves. If each brings you 2 gold you are break even.
Micro-manage cities for gold and science production:
I am rather tardy with this, and tend to do it haphazardly (its cumbersome). But ideally, it needs to be done.
So open up the citizen screen (especially for all your weaker cities, cities on the coast, cities captured from the AI later and cities that should not be expanding further) and manage the citizens. Normally, I put at least one citizen as a specialist in the campus district, for weaker cities, ignore the production and put the rest on gold tiles (if ay). There may not be many gold tiles (map-dependent) but often there will be some. Its ok if these cities dont have any production.
Maximise gold from Trade Routes:
Obviously, as in the original post, almost all traders need to go to commercial city states. Save 2-4 (depending) to grow/supplement your capital (or spaceport city) and your Big Ben city a bit. Be strategic with the first few trade routes you send - you need to ensure these will place some good trading posts to expand coverage. Use the continents lense and try to maximise the number of traders not on your original continent.
So how many traders do you need:
Depends on all of the above.
In best case scenario - with lots of Commercial CS and higher difficulties, you could get by with just 8-10CDs and a few captured harbours for a total of 12-14 trade routes. On lower difficulties, you definitely need more (close to 20 traders) (so around 15 CDs).
If you go for Great Zimbabve, then likely less (but I'm still yet to do a GZ sub 140 victory).
How to play your cards policies:
This is a tough question. You need to find the balance. It depends on the difficulty, overall number of traders, gold you get from AI or otherwise, wildcard slots etc and importantly your science/culture progress.
In general, never use the +2 gold card. Its just not worth it.
Certain cards are a must mid game: (+1 culture for every district, 100% science from campus district buildings) always. The 100% science adjacency bonus card depends on the map. If you have good adjacency, keep it. If not - dont. Do a quick ballpark: how much extra science per turn does it give? How much is that in 50 turns? On some maps, I keep it all the time. On some maps, I ditch it because at one point (e.g. turn 100 or so).
Envoys? How are you going with city states? Are you in need for envoys. In the early-mid game the (first envoy counts as two) and (+2 influence points) are essential. Late game, definitely take out (+2 influence points) (normally i slot in the +2 each for every city state you are suzerain). If all is going well, for some time, your main (and often only gold card) could be the (+1 gold from each envoy). Find the balance depending on circumstances.
So in most games, unless you're trying to compensate science etc, you could slot the (+4 gold +1 faith card). At one point (if its worth it on the map) slot the 25% gold from cities not on your original continent.
In any case, don't take any of the advice above to heart. These are all general tips (lines of thought) but the key to a quick SV is adapting your gameplay to the map and circumstances. And importantly not wasting gold and production on things you dont need.