How to get Population above 12

[ArthurBede]

I cannot get population in any cities above 12. what are the requirements?

Thanks

[Mace]

You need to build a Hospital in each city you want to get above size 12. To build a Hospital, you must discover Sanitation first.

Also make sure the city can support > size 12; a city in a mountainous area, for example, may never be able to reach size 12.

Also, above size 12, your cities start requiring a lot of extra things; (read: expensive things)..such as mass transit, cathedral/colosseum, etc. If you are playing a warmonger game, many players leave the town at size 12. If you are playing a builder game, or going for culture, etc. then by all means let them grow as large as possible.

[Rhothaerill]

Research sanitation in the Industrial age and build a hospital.

EDIT: Crosspost. Mace's response was better anyway.

[ducki]

Or build Free Artistry for a one-city ability to grown beyond 12.
Edit: Er, I mean Shakespeare's Theatre, which becomes available with Free Artistry.

[vmxa1]

You will also need to be producing a food surplus to grow after you get the Hospital or the Shakespeare wonder.

[MrWhereItsAt]

Keep in mind before allowing a city to grow beyond population 12 that when it does, population-induced pollution starts to really cause a strong effect. If you are not ready for it the larger city size can give you substantial headaches. This is fine if you have enough workers nearby to clean things up quickly, but otherwise it can cause you to lose the population you were building up in the first place!

Also a city can work only 20 tiles (21 if you count the 'free' centre tile), so any population beyond 20 can only become a specialist.