I do not believe we have any limits in terms of how many times you can `cf ssh` into an app container. I did a quick test and was able to ssh into an app from three different terminals all at the same time.
A timeout like what you're seeing sounds more like it could be network related though. I would suggest maybe trying from a different network, like a mobile network or from home, assuming you're in an office network. Office networks can be particularly difficult depending on what type of network gear is installed and inspecting outgoing traffic.
Hope that helps!