It doesn't have to be a distributed setup. Previously ASCS was installed for cluster installations.
I have seen distributed setups without ASCS.
ASCS is now part of all installations whether it is distributed, cluster or standard installation.
However there is no hard n fast rule of message server port to be -1 than instance..in distributed system it can be assigned to any instances or dedicated.
The ASCS will run separately and has its own instance number. The message server is part of that and port number is calculated based on the ASCS system number.
Regards
RB