There is a way to add limitations on parallel jobs in DSS.
If you check in Administration > Settings > Flow build:
In the additional limits, you can add a limit for tag/gpu to 2 for instance.
Then on the recipes from the flow, you can add the gpu tag on the recipes using gpus.
If you do that, it would mean that you can't start more than 2 recipes using gpus at the same time in DSS.