How many cron jobs are too many?

I have a couple of cron jobs for basic maintenance which aren't very resource-intensive.

I also have custom task scheduling (which is just calling a .php file and passing information via GET, ie: cronjob.php?param1=param ...). These can add up pretty quickly.

These just call system commands and run external programs (Nmap is one of them). They usually don't take long either.

Anyway, can anyone tell me, roughly what point is too many? I know it's hard to say since it depends on what job is being run and how often, but at what point does the crontab program start "struggling"? Anyone have any idea?

Thanks.