1) How do I get science improvents at 1 every 2 turns (like one dude was talking about), and 2) how do I supervise 30 or more cities? Its all too mind-boggling for me..
You cannot do 1 every 2 turns at any old time. You have to prepare, and I do it mainly with an SSC and caravans in mid game. Somewhere around Corporation-Automobile, I like to push for 1 advance per turn, and sometime around Computers, hit 2 advances per turn to finish off the tech tree.
How? The SSC, Democracy, and trade routes + trade bonuses. You have to make a steady supply of freight, and deliver it to the best locations. To get the most value, it is essential to have big cities (like about 20+) with all trade enhancing available improvements. In a republic, a courthouse is a trade enhancer in most cities. Sewer systems enhance trade... without it, a city is stuck at size 12.
Planning trade and fast science does not suddenly begin at RR or Industrialization... it is cumulative effort, and begins with the very first city you found. One technique is to use Monarchy/Communism/Fundy to build the infrastructure to support a democracy (mkt, tmpl, bank, stk, Michelangelo, JS Bach, courthouse). Get into Democracy by about Corp. Build Cure for Cancer. Then SHs and Airports.
You have to look at your exact science output (see trade advisor), and cost (multiply turns per advance times science output). Deliver caravans/freight to boost your science (see sci advisor) until you can complet an advance on that turn.
Later, when you can jack your science up to 60 or 70 or 80% and get one advance per turn, then ALSO deliver enough freight each and every turn to bring the science advisor to a full bar. This will give 2 advances per turn.
2) how do I supervise 30 or more cities?
Get into democracy and raise luxuries (also build mkts, etc.). Or use Fundamentalism if you cannot yet control an young democracy

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Personally, I go Monarchy ---> Republic ----> Democracy (no more govt changes after Democracy!). Sometimes, once in a while, I use Communism instead of Republic.