It depends what you mean by "Captive Portal". If you're referring to Integrated Guest Portal (where the user credentials database is the controller itself), then as Ronald mentions there is only one instance per controller.
However, the other Captive Portal configurations (Internal Captive Portal [radius authentication of users], Guest Splash or full External Captive portals) don't have that limit. You can have a unique configuration per WLAN service.
For user authentication you'd need access to a Radius Server [ NAC or any other RADIUS server implementation]
