Device A can connect only one at a time to service S on Device B. Device A can connect to service S on Device B, C, D and E, etc at the same time.
In details a RFCOMM service listens on port number and publishes that port number in a SDP record listed by UUID. Unlike TCP/IP which uses source and destination port numbers in its packets RFCOMM only uses one port number in its packets and therefore only one connection can be active to that port number between a pair of devices.