In fact, in a 11 pull, the probability to obtain one (or more) 6* is 0.5*11, so 5.5%. The probability of 11 6* is 0.5^11=0,000488281% (1/2048 %). So you just need to do some hundreds of thousands of pulls to have a high probability
The probability of 2 6* should be 0.5%*11 * 0.5%*10 = (0.5*11)/100 * (0.5*10)/100 = 27.5 /(100*100)=0.275% . And I had one some time ago
