hi sd isaac,
As it seems, PSR-S970 doesn't have option for showing remaining user memory capacity (which is probably the reason why you ask). Anyway, I'm sure that in case there's not enough space left, you will be warned when you attempt to save a new style (or anything else).
13MB isn't that much space.. or can be just enough -depending on how you organize things. If average style size is about 30kB, then there's space for ~450 styles.. sounds a lot, but it isn't really. The thing is, we usually save all kind of things into user memory (voices settings, registrations, etc.), which also eats available space. And finally, 450 styles is only a theoretical number which probably can't be achieved (depending on what kind of file system Yamaha uses).
Anyway, in such case, I would use user memory for "must have" things only and the rest I would save on USB stick. Well, I guess I told you nothing new really.
Bogdan