As a general rule of thumb, you should have a bankroll of at least 20-30 buy-ins for the particular game you are playing. For a sit'n'go with a buy-in of $2.50, that means you should have a bankroll of $50-$75. This will help ensure that you have enough money to weather any downswings and will give you the best chance of being profitable in the long run. However, it's important to note that this is just a general guideline and you should adjust your bankroll requirements based on your personal playing style, risk tolerance, and the specific game and stakes you are playing.