I started off buying 10 dwarves and it worked out for me, I feel there is no minimum.
Solve a few games to save CPU time, and then use that to buy dwarves any Global Index is under 100.  The lower it is, the much better return probability you have.
I bought a 100 dwarves recently for 1RNG , 3 times, and all of them turned green before the 10 days was up.  I know have enough to buy 5 sets of 100 (5 RNG), so almost double what I put in.
That was at a global index ranging from 49 to 81.
I feel this is a great balance of investment, puzzle solving, and CPU mining.