Hi and welcome to BYC! As far as a terrarium as a brooder..... it will really depend on how big it is, how many chicks you have, and how long they'll be in the brooder before being moved to a coop. Brooders don't have to be complicated-I brooded my last batch in a kiddie pool with disassembled cardboard boxes for "walls" with chicken wire over the top once they were big enough to fly out. My next batch will be brooded in a makeshift cardboard box container while we build a new coop. Baby chicks (likely less than a week old from TSC) grow *fast*. You will not want them in your house past 4-5 weeks old. As far as how many, it depends on how much room you can provide them in their coop and run space. Commonly quoted space requirements are 4sq ft per hen in the coop and an additional 10 sq ft of space per hen in the run. Personally, I find this to be good advice in general. Are there situations where more or less space make sense? Yes. But for most situations this is good advice-especially in smaller flocks of hens only.
My first flock was 6 chickens. My coop was 30 sq ft (5x6) and they had a 60 sq ft (6x10) enclosed run. They were allowed to free range in the yard each evening and most weekends. It worked out fairly well although more space would have certainly been better at times.
My advice would be keep it at a level where you can provide roomy and predator proof accommodations. Manufactured coops that you can buy at TSC (or similar) always overstate (by large margins) how many birds you can have. You don't want to be building your first coop with 2 week old chicks in your basement-don't ask how I know this
Take your time, have your coop ready, read up on predators and how to safeguard your chickens, and go from there. Good luck!