It Depends (tm)
You don't say what operating system or what version of cron you're using so its hard to know for sure.
Did you check "man cron" or "man crond"?
I believe that with some, there is a limit of 256 entries.
A quick test on Solaris suggests that a user is limited to 100 lines (but all entries continue to show up in 'crontab -l').
Why are you asking what the limit is? Starting all of those processes automatically at the same time sounds like potential trouble. :-)
To keep my own sanity I prefer to create cron scripts called Hourly, Daily, Weekly, Monthly. etc.
This allows me to group commands and make sure everything doesn't fire at once.
Depending on your needs, this might be a good solution for you as well.