Zoom allows the Host (or co-Host) of a meeting to assign any Participant to type closed captions. More information about using Zoom in an ELMS-Canvas Course can be found at Instructor’s Guide Zoom and Student Guide for Using Zoom.
Only one Participant can be assigned to type closed captions, so only one breakout room can have closed captions. The Host should assign the closed caption typing role to a Participant before starting the Breakout rooms.