I'm not aware of any scalability issues.
While a sequence does not perform any locking there are of course soem
short term latches. If you have an extreme(!) concurrent use of a
specific sequence  it that latch could, theoretically, run hot.
That woudl be a case in point not to reduce the number of sequences.
But again.. that's extreme. I doubt there is an app out there
experiencing such issues in reality.

Cheers
Serge