One page sites

[Halfdeck]

I read some where recently that you get penalized if you have more than 100 per page.

Google doesn't have a set limit on links/page, though they used to have a page size/link limit due to server capacity. Links/page is a quality signal IMO, but having 300 links per page in and of itself won't harm your site. On the other hand, if your 300 links/page site are propagated across 1,000 subdomains then you might be pushing it a bit.

Is Google on to one page sites that have 10 Thousand link trades at the bottom?

Absolutely. That's a nice example of link spam, a ticking timebomb ready to go off any minute.

Pages like that not only hurt its owner - it pulls every one else in that link scheme network into a bad neighborhood.