All depends on what is allowed. My local casino, Tioga Downs, mostly runs $200NLH Match Stacks so the limits are initially $300 max buy-in but once the game has been going, you may buy-in for whatever the largest stack is. I plan to have at least $1500 with me when I go play and I have bought in for that $1500 a couple of times.
Many casino poker rooms have a fixed maximum. The traditional max is 100 times the BB, so it would be $200 in a $1-2 game. Always sit down with the max, or you will be targeted by larger stacks who can afford to play marginal hands in an effort to bully small stacks.
Usually 50-70 BBs for 1/2 and about the same for 2/5 because I'm taking shots and I like to start off playing a simpler short stack game as I learn what the table is up to but I'd likely always buy in for max if I was sufficiently rolled.
I usually play only 1-2 tables and wont buy in for more than 100 to start. I like to see how the other players are playing first. I don't wanna sit down with 500 and get taken to school the hard way. If I fell good after sitting for a hour or 2 I will buy in for more, but likely I am ahead at that point and don't have to buy any more chips