All 5 of them.
It was full to the point where I had to use my donator slot yesterday. Also wanted to point out that most are still at school/work right now so none of the servers will have that many people.
On a semi related note:
We're working on a mandatory update for TF2. We should have it ready soon.
-Eric
I hope it's the main Halloween update and that it doesn't break everything again.EDIT: It turned out to be nothing, just a few bug fixes. It also didn't break anything. This is a first for Valve, and we must mark this day as a major milestone for them.