A rough way of calculating it would be to consider each selection of a data point to be a separate random event. So, you have a 20% chance of choosing a "missing" data point (a 1 in 5 chance). If you were to hit that 1 in 5 chance 12 times in a row, you would calculate P=.2^12=0.0000004096%.
If you wanted to be more exact, you would have to account for the sample taken out of the population each time you select one. If the first time you have a 20% chance (115/577=19.9307%), the next draw, your chance will be slightly lower (114/576=19.7917%). And the next time, even lower (113/575=19.6522%). If you were to follow that logic over 12 draws, multiplying each probability by the next, your total probability falls off a little to 0.0000002434% (to 10 decimal places).
I hope that helped a little (and that I didn't make a gross mistake anywhere).