One right now, our custom one. We'll soon be adding a dark theme and then one additional one voted on by members each month. That will be it until we add another custom theme or something.
Though 3 of them are basically the same skin just with different colour variations (Gold being the default colour, Blue was the second iteration and Purple was the latest).
We have a theme based off the character Vile from MegaMan X (returned by popular demand from our previous skin set).
We have a theme based off GamingForce's Revolt (Dark) theme. (Which you should go check out GF's E3 coverage...)
We have the default MyBB theme (which no one really uses).
And then finally we have our mobile theme. (Which I really haven't customized or modified since installing the mobile theme plugin.)
For most of my forums, I always tried to have at least one dark and one light theme available since people usually have a preference between those two.
Just two: the main custom theme and mobile version of the same theme. I don't see need for more as I'd say that the main theme has a good compromise between dark and light.