Quite a few, but you need to consider tyre size.
Certainly a lot of the late 00's mtbs with sliding dropout configurations should fit 27.5 (with dropouts slid fully back).
But I wouldn't expect you can run much over 2.25" tyres on most, due to the chainstay clearance at the seat-tube end. Plenty for most people, but a lot of the modern mtbs are running 2.4 - 2.6 and beyond!