There are no limits.
Just that usually fruits tree are pruned with maximal 3 (some time 2 to 4) main branches, so if you do graft more cultivars, you should have trouble on giving the right form. [With "espalier" form you could have more, but this system for commercial apples requires many trees]
The optimal is often zero grafting: this is the best way for plant longevity and fruiting. But it is nearly impossible to have the right variety from seed.
The second best is to have a single grafting: this allow to clone a cultivar, and possibly to choose the root stock so that it is ideal for your soil (acidity, dryness, wake up period in spring). But in this case, all leaves (and sprout) of root stock should be removed, or they will dominate.
As you see, in the last point, more than one grafts is usually bad. Usually are just lure of gardening shops. All branches are competing (this is in general, also on non grafted trees), and so we have pruning, not to have just good branches very high (and vertically). With multiple grafting, this would be a nightmare.
But there are some case that multiple branches are useful. Kiwi and apples are good examples:
Kiwi needs male and female trees, and grow very wide, so just one tree with one male branch is good (and usually simpler to prune, because pruning is done horizontally: easier to control apical dominance).
But also apple are often (for garden) grafted twice, because of non-autofertility (fertilization, so apples, is successful only across different varieties). But often this is done like kiwi: one for good fruits, and one just to provide pollen.
Because you can have small apple trees, I would recommend to plant few different apple trees. Look for root stock which keep apple trees small. Pruning and pest control will not become a nightmare. (and you can get your apple by hand, with more grafting you should also go vertically).
