I just wanna say, counting the tiles is not necessary at all. Wreck may have a good point that an "ideal city" has size 20 and no surplus

, but I would argue against it. You have to consider what "you need atm" . If you're running Slavery i. e. (as you probably will for most of the game) you'll want a good

surplus to speed up production. Once you have your cities set up you still might want to need that production to produce military units, but it might also already be the time when Civil Service comes in reach, so you'll maybe wanna build some Cottages so those get multiplied. You might wanna do that even earlier, because they need time to grow and your expansion force you into building them.
Therefor I stick with these rules:
Improve the

first, after that, decide which specialization the city will get.
-> Military city: Build Farms
-> Military city with HE: Improve Mines
-> Commercial city with

surplus greater than 4: Build Cottages
-> Commercial city with

surplus smaller than 4: Build some Farms to speed up growth
-> GP Farm: Build all Farms
If you are still very early in the game and your

cap is low, improve the mines before building too much farms, given you improved the

and have a decent surplus for the city to grow.
Remember: A "real city" will never reach size 20 in a decent amount of time, it will stay small and "work only the best tiles" . I. e. your capital has an "ideal size" of 15. You will first want to improve the Corn, that gives you a

surplus of 5, which is ok, but not ideal for the beginning. Therefor you'll want to farm 3 FPs to produce fast Settlers at size 4. When your

cap rises, you'll want to cottage those FPs and you'll want to mine those hills, as both will get multiplied by CS. The extra

from the Bananas aswell as the extra

from the dyes will come in handy shortly before that. You'll end up with all FPs + riverside grasslands cottaged, a

surplus of 6 (3 from the Corn, 3 from the Bananas, 2 from the initial tile and -2 from the grassland hills) , so that city will have quite good

, is a little low on

but makes up for that because of the good

surplus of 6, so prepare to whip most of the Infrastructure it'll need, but don't whip away good tiles. As it's a Capital, actually only whip the granary and the Library and the initial Settlers, after that let it grow because you'll want it to be size 13-15 asap.
If you got questions about how "real cities work" , post screens of the cities and state the situation, you can follow the rules above and you'll have a better outcome than you'd ever had, if you'd blindly follow the rules about the farms and size 20 you have learned, they lead to "first build all the farms" and "win too late" .
Regards, Sera