Probably more than you think!
Check out this video on apple tree grafting from the Turkeysong Experimental Homestead :
So far he’s grafted over 140 varieties of apple onto one apple tree.
Do you have a lousy fruit tree growing in your yard? Perhaps a wild fruit tree? A native persimmon that doesn’t fruit or a crabapple that produces fruit so bad you wouldn’t feed it to a pig?
Look at that tree as a rootstock for grafting experimentations. It isn’t hard to graft. Anyone with a few tools and a little time can do it.