With today’s BMv2 source code, it requires modifying it and recompiling simple_switch and/or simple_switch_grpc in order to enable more than one FIFO queue per output port. Without changing the source code, there is only one FIFO queue per output port. See this Github issue for the (small) source code changes required: Does simple_switch_grpc support priority queue ? · Issue #877 · p4lang/behavioral-model · GitHub
This later Github issue refers to that earlier one, and proposes making an enhancement to the BMv2 code that would (after those enhancements were made) allow users to enable multiple priority queues per output port via a command line option, but so far no one has volunteered to make those changes: Consider making it a command line option to enable priority queues · Issue #1065 · p4lang/behavioral-model · GitHub