Which interface on the WLC is limited to one when LAG is in use?

The service interface is an out-of-band management interface whose limitation isn't directly tied to LAG in the same way as the AP-manager interface.

The virtual interface is a dummy gateway for client and AP communication, and its configuration isn't limited to one instance due to LAG.

"Trunk" refers to a port mode carrying multiple VLANs, not a specific WLC interface type that is limited to one when LAG is active.

When LAG is enabled on a Wireless LAN Controller, the AP-manager interface is limited to a single logical interface, which is then used by all access points to join the controller through the LAG bundle.