It's mentioned in their Webmaster Guidelines. Do not use more than 100 links on a page. But that's total links--internal and outbound. You might want to keep the total number of outbound links to a much smaller number. "Keep the links on a given page to a reasonable number (fewer than 100)." http://www.google.com/support/webmasters/bin/answer.py?answer=35769
It's not really a restriction, but I find that I usually see a PR degradation at around 12 outbound links per page.