To quote from the SmashWiki:
The boost applied by Revenge is equal to 0.075 * d + 1.25, where "d" is the total damage of all attacks that have been stacked, with an upper limit of 3.0x. For example, using Revenge on an attack that does 15% damage (or three attacks that did 5% each) will apply a multiplier of 2.375x.
When Incineroar uses a move with Revenge active, each different kind of attack has an additional knockback multiplier if it hits, and number of seconds lost from the boost's storage if it misses.
Also, yes, it's possible to deal 999% provided the right spirit setup. The easiest one I can think of would be to have an Incineroar counter a revenge-boosted move from some strong move, say, maybe Warlock Punch or some elaborate OHKO setup, before unleashing said move on a small target with a weak spirit. I recall Alpharad doing a setup with a Ganondorf, two Incineroars, and a Shulk. The first three were giant, the Shulk was tiny, and he had Buster active to maximize the damage dealt. The old setup involving Game & Watch's Oil Panic being countered numerous times by Buster Shulks should still work, but a setup with Revenge will definitely work.