If topology.acker.executors is not set, the number of ackers will be equal to topology.workers. But on RAS cluster, we don't set topology.workers because the number of workers will be determined by the scheduler. So in this case, the number of ackers will always be 1 (see attached screenshot)
The number of ackers has to be computed before scheduling happens, so it knows how to schedule the topology. The number of workers is not set until the topology is scheduled, so it is a bit of a chicken and egg problem.
We could probably use the total amount of requested memory when the topology is submitted divided by the memory per worker to get an estimate that is better than 1.