26” frame, 27.5 wheels?

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!
 
All 26 can fit a 27.5 “ with 650-42 tires … because they are near equivalent. If you buy some rimbrakes wheel set 27.5 you have to move the brakes too.

This is mine, a 27.5 gravel tire in a 26” frame. Lot of clearance:
592216E4-905D-44F8-B4C8-3F549594682A.jpeg
 
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