[AaronBrown]

It always pays to start simple, then add complexity if necessary.

Suppose the only question were to determine the probability of being in the money. With 10 players, 30% will be in the money. After 500 hands, the standard deviation of times actually in the money for the average player is (500*0.3*0.7)^0.5 = 10.25. With 6 players, 50% will be in the money, so the same calculation is (500*0.5*0.5)^0.5 = 11.18.

With 10 players, the average player expects to be in the money 150 times, 10.25/150 = 6.8%. With 6 players it's 11.18/250 = 4.5%. So it would take 500*4.5/6.8 = 327 hands of six player tournaments to get the same relative error of chance of being in the money as 500 ten player tournaments.

Although ROI is a little more complicated than probability of being in the money, there's no obvious reason to think it changes the relative information based on sample size. If you think it might, it's possible to make a more complicated model, that could give you a more precise result (unless the assumptions are wrong, in which case it might do worse).