Increase concurrent users by limiting container resources with docker-swarm Consider the scenario where I have:

4 replicas of an app
4 cores
On load, the app consumes 100% for a single core

Can I tell each replica to only use 50% of the core and if so, would this allow at most 8 simultaneous connections to the app? I have just dipped my toe into this stuff so an feedback would be appreciated.
In general: No you can't. At least not on the Docker level.
You can maybe code your app in a way that it monitors such metrics and adjusts itself but Docker isn't able to.
What Docker CAN do is limit the app to 50% and kill it if it uses more than that by utilizing resource limits:
docs.docker.com - containers/resource_constraints
docs.docker.com - docker-compose resource limits