The size of the minimum rate must be chosen equal to no more than 1/2000 of your bank. Why 1/2000? Because you will have a margin of 9 losses of the generated number in your chosen range and balance balance for the 10th rate, after which about half of the balance remains.
If you mean betting with 50% win chance, then even 1/2000 of your bank can be not small enough. Losing 15 times in a row is a common thing, and over 20 times in a row can happen too. The thing is that if it was possible to say that you are insured against losing over 20 times in a row while betting with 50% win chance, many people would just be rolling on dice sites instead of mining.
