According to the Cat4500 configuration guide, the number of sources is only limited by the total number of ports or VLANs on the switch:
http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/54sg/configuration/guide/span.html#wp1043675
The switch supports any number of source ports (up to the maximum number of available ports on the switch) and any number of source VLANs.
Keep in mind if you oversubscribe the source to destination, there is a chance you will see overruns on the SPAN destination interface.
Hope this helps,
Matt