If you search back, you should find this subject has been covered a few times.
Basically a family of engines will be given a nominal setting for 100% fan speed (e.g. 3900 RPM on RB211-22, -524).
This is based on the original design parameters for the fan (e.g. Tip relative Mach no.). Percentage rather than actual RPM is used to give a normalisation of presentation between different aircraft/engines.
Actual RPM limits can vary with different variants but will usually be slightly higher than 100%