Has there ever been a tournament with only one number 1 seed in the final four?

Yes, but it is not common. So far it has only happened three times:

1980 Louisville #2, Iowa #5, Purdue #6 & UCLA #8

2006 UCLA #2, Florida #3, LSU #4 and George Mason #11

2011 Butler #8, UConn #3, Virginia Commonwealth #11, and Kentucky #4