I am pretty sure there is no limit to the length of a cron file (maybe 65536 chars/lines due to 32bit limits?).
Some people on Stack overflow and Server Fault thought there was a limit:
(2011) Ubuntu Debian is limited to 256 (and I just ran into that). FreeBSD will happily accept 600 entries. I haven't checked beyond that, but 600 crontab entries ought to be enough for anyone :)
But that wasn't the accepted answer, this was and it says there isn't one.
There doesn't appear to be a documented limit to the number of crontab entries allowed. So, short of checking the source code to the specific version you're using, perhaps the BUG entry in the crontab manpage could be helpful:
Although cron requires that each entry in a crontab end in a newline character, neither the crontab command nor the cron daemon will detect this error. Instead, the crontab will appear to load normally. However, the command will never run. The best choice is to ensure that your crontab has a blank line at the end.
And on Unix and Linux, this was the answer:
cronies limit seems to depend on the filesystem (maximum file size) as the cron daemon stores the crontab entries per user in a linked list - see user.c from cronie for details. This means the maximum number of job entries for crontab is basically unlimited.
So I'm pretty sure that it is unlimited, but you need to have that blank line at the end. :)