It's all depends what is the max bet your are going to offer, you need to have atleast 20 times more bankroll then the highest amount of money that someone could receive from one roll.
For an example, if maximum bet is 0.1btc on 10% roll, then you should have something around 18-20 bitcoins in your bankroll so in case of someone that bet max and win you won't have a very decent loss, and also you will be able to cover a large amount of max-bets in a case of bad luck.

Always make a spread so you won't feel that you are dangerously close to losing your whole funds.