It's not something defined in the documentation. I doubt that anyone knows, unless they've run into the limit.
Bob doesn't know the answer and neither do I. Why don't you try adding master pages until you hit the limit, and you can tell us.
If I remember rightly, there's a 9999 page limit, but that seems NOT to include master pages (I just checked). Like the others, I've never seen a documented limit, and I suspect that for all practical purposes you could consider it to be more than you are likely to ever need in any document.