Probably 255 (because of bitmasks maybe?). That’d make sense, sort of. But how are you even going to use it? Choosing something from a list of 30 items is already annoying. 300 would make it so much worse. Esssentially, enumerators are not meant to be used like that.
Imagine hooking up a Select node or Switch to a 300 element long enum. Not only would that be exceptionally awkward to use but also quite inefficient due to lack of close circuit evaluation in the engine. And likely to come crashing down.
You may need to use names instead or rethink the approach.