One way is to maintain a counter file that is checked every time a script is run. If the counter file holds less than five, go ahead and increment it by one and continue execution. At the end of the script, decrement the counter file by one.

This can get messed up if people kill-9 one of the scripts.

One way to deal with the possibility of active processes being killed, thus fouling up the count file, is to use timestamp entries in the countfile. If you know that there is no way your script should take longer than 1 minute to run, and upon script startup, you find an entry in the file that is ten minutes old, remove that entry; it's a ghost.