The thing to consider is while running each of the terminal setups is possible, each additional instance will use more and more of your computer's resources to do. That could affect system performance if you get enough copies running at the same time. Another possible solution some people work with is what is called a VPS (virtual private server) and run one EA on each instance they have. If you are just trying to test, there is also also optimization testing available, although I am not sure if it is only available through MT5/MQL5 or just 4 or both. There is a cost to this but it can help out with the optimization a lot quicker than just running it on your system.
Edit: I don't remember if it was on 4 or 5's forum, but there was a discussion there about how many of all of the different things that could be used, like how many charts maximum, etc. Not sure what all of the parameters were, but a few of the things that I remember from that (and other sources) is the most charts you could have open at once was something like 100 on one running program of the trading platform, and the most EAs you could have per chart was 1. You can subscribe to lots of signals, but I do not remember if they said you could only have 1 active at a time per account.