At my group's table, there's not really such a thing as too much. There's probably a hypothetical limit out there, but I don't recall ever coming close.
For example, my friend is running a post-apocalyptic style fantasy game. He threw out all of the existing classes, races, and most of the weapons and homebrewed his own. It's all new stuff but it's still running on the 5e core. My Roach Priest still casts cleric spells. So on and so forth. For us, it's still 5e (albeit heavily modified), and we're having a blast with the campaign!