Docker has a --ulimit flag that can be pass when starting a docker container. We are running the centrifugo/centrifugo:v5 docker image (public) to handle push notifications as a docker container on render. The centrifugo docs strongly recommend setting this to a higher number at runtime to scale.
We don’t have this capability today. I’d encourage you to submit a feature request at https://feedback.render.com/. It helps to include as much context as possible about your use case, the problem you’re looking to solve. For example, beyond just the fact that the docs make this recommendation, it would be useful to understand what problem this solves for this particular image.
We rely heavily on customer feedback as a part of our planning and product roadmap process, so capturing interest on the feature request page is very helpful.