How many votes does a wish need for it to be purchasable?

What's purchasable or not depends on whether GOG can get the license to sell it. The voting in the wishlist is a good way to let them know what players want relatively more, but it doesn't help with making those games purchasable.

2 is enough but 39710 is not.

A better way of putting it is that the voting system gives GOG both a way to prioritize which games to focus on trying to get the license for first and to know how much to invest in that effort.

Thanks.