The number of chips you start within poker depends on whether or not you are playing a cash game or a tournament. In a cash game, the amount is determined by both the minimum and maximum buy-in. Most commonly, if you were to buy in to a $5/$10 online game at Black Chip Poker, for instance, the minimum amount would be $400 and the maximum would be $1,000. If you wanted to play the same game at Global Poker, the maximum would be the same, but the minimum would be $250.
As for tournaments, the number of chips you start out at is predetermined for all players, but is normally between 2,000 and 10,000, with the exception of deeper stacked events, which will often be between 20,000 and 50,000.