If you’re on a 32 bit system it’s 2,147,483,647. On a 64 bit system, 9,223,372,036,854,775,807. You’ll probably run into filesystem limits and other scalability problems (such as the universe ending) along the way to creating that many versions.
There is also the question of what syncthing requires in terms of hardware configuration and network speed
Depends entirely on the amount of data you’re going to throw at it.