If you do a search you will find this question answered.
Briefly, 100% N1 is an arbitrary figure based on the design tip Mach no for the fan usually, but often rounded for convenience (e.g. 3900 rpm on an RB211-524). It allows some commanality of instrument presentations but does not imply a limit.
There may be RPM limits specified for stress or vibration reasons but more usually you will be limited by temperatures etc.