There isn’t a set limit; however there may be problems when someone has 40+ windows open of any kind. There is a limit on the number of markets you can get quotes for at once – this is currently set to 2500. If you exceed this limit then the market mode for markets over that limit will be red and say “Rejected”.
If you load too many windows and run into any issues this will be due to insufficient Windows resources. This is not the amount of RAM in your PC. You can try either reducing the number of windows you have open in T4 or you can edit your computer's registry to increase the number of Windows resources available to each application. Please note that editing your computer's registry may cause system instability or failure and is undertaken at your own risk.