I've never tried to populate a town.
However, it seems the average town population give or take is around 100 total Sims. Maybe start with 50 Sims, get a feel for that. If you want more then add more. Do so in small amounts, like 10. So as to not over-populate. Easier to add then it is to take away.
If you play with any kind of story progression it will add/remove Sims as you play, so that's something to keep in mind.