The number of available VLANs on a Corebuilder 3500 is a function of the configuration. If you have six, twelve, eighteen or twenty-four fast ethernet ports the number of VLANs available does not change only the number of ports on the switch. (There are some limitations outlined in the implementation guide)
There are usually 64 VLAN entries available, if you configure 12 layer 2 or layer 3 vlans and subsequently add two more FE modules you merely add available ports not VLANs the 12 configured VLANs are still all you have configured and have available. You can still create up to 64 VLANS. Note that if you add modules out of sequence the port numbering changes, so for example if you had slots 1 and 4 populated and inserted a module in slot 2 the port numberings would change but the VLAN allocation for any given port would not. e,g if port 7 was in VLAN 2 after the module is added the port number changes to 13 but it remains in VLAN 2.
I refer you to the implementation guide which explains all of this.
