I believe there is a limit, but can't remember what the correct number is. The limit has to do with the number of PTY's (pseudo terminal devices) that can be allocated for use at one time. Each login session runs on it's own PTY. Some scripts create their own PTY's to run on as well (for instance, anytime you "spawn" a program in Expect, it opens a PTY for that program.) I know there's a limit to the number of PTY's that can be opened, because you can open all of them very quickly by repeatedly spawning programs in Expect without doing a "wait" for them to close before spawning the next program.