There are limits imposed by available memory, bandwidth, CPU, and of course, the network connectivity. But those can often be scaled vertically. Are there any other limiting factors on linux? Can they be overcome without kernel modifications? I suspect that, if nothing else, the limiting factor would become the gigabit ethernet. But for efficient protocols it could take 50K concurrent connections to swamp that. Would something else break before I could get that high?
I'm thinking that I want a software udp and/or tcp/ip load balancer. Unfortunately nothing like that in the open-source community seems to exist, except for the http protocol. But it is not beyond my abilities to write one using epoll. I expect it would go through a lot of tweaking to get it to scale, but that's work that can be done incrementally, and I would be a better programmer for it.
To your question, you are only restrained by hardware limitations. This was the design philosophy for linux systems. You are describe exactly what would be your limiting factors.