Agree with Drake, as many as you need; but I'm in a unique position, not only do I need one for each song, but also one for each platform I'm demoing on, if possible (e.g. a Tyros 1/PSR3000 version, a T2 version, an s950/CVP605 version, a T3/CVP509 version, an s970/CVP705/sx700 version, a T4/CVP609 version, an sx900/CVP805/905 version, A T5/CVP709 version, a G1/CVP809/909 version, and now a G2 version.
So as many song styles as you need, but also as many versions as you'd need. For example, in most cases I wouldn't need a T4 *and* an s970 version if I just use the voices and drums common to both.
Because ask Charley (Rattly) said: while all these styles sound great on G2, if they're not *optimized* for G2, they don't sound as great as they should. Same goes for styles optimized for G1. One of my G1 customers requested a style, I made several versions, and let him try them, and unsurprisingly he liked the one designed for his G1 the best; the other versions were *similar* but really *not* identical.
Even my latest Final Fantasy Youtube video I have T1, T2, T3, T4 T5 and now Genos version (there was literally *no* difference between the T4, T5 and G1 versions so I skipped those; because all 3 have *identical* choir voices and use the same symphony drum kit, which in turn has been in use since the T1. (but there are huge voice differences between the T1, T2, and T3 regarding the accompaniment voices used).
Mark