20 Buy-ins for whatever game you play is the standard recommendation.
TeUnit makes a good point that this depends on the game you are playing. The original post seems to be asking about $1/$2 live. 20 Buy-ins is the typical recommendation for this game.
As some posters have implied, most players are not winning players over the long term. So for most players, the answer to this question:
"What size bankroll do you need to win at poker?"
is that an infinite bankroll is not enough.
If you are not a winning player yet, play the smallest stakes you can find, don't bet more than you can afford, and make sure you are spending a lot of time studying poker.