who know how many users is possible manage with WLC 5508 ?
Can the finally users modify his password at the first loging or later ?
the password change is a feature that you have to look for on the authentication server (radius). The radius server that resides on the WLC doesn't have all the features that ACS can have for example.
The 5508 supports up to 500 access points. The client table is limited, I think, to 7000 clients (it was 4000 for the 4404).