I spent many an hour crammed into the jumpseats of 727's (they have 2, both in the broom closet sized cockpit with 3 crew members). I worked as a cargo handler at the FedEx hub in Memphis and at the time we could book any open jumpseat.
When i clocked out at 4am there were a couple hundred planes just about to depart for destinations all over the world. It was awesome, but alas, no more. It even continued after an off duty pilot with a hammer decided to try and hijack one of our planes from the jumpseat. But 19 jerks ended all that on Sep 11, 2001. There are still non-crew persons in the jumpseat but now you have to have a good reason to be there. It's mostly deadheading pilots but anybody that has an "operational need" for the company can jumpseat. So if they need a mechanic somewhere or something like that they can go.
As far as what jumpseats there are it differs by aircraft. Like i mentioned the 727's had 2 terribly uncomfortable seats in the cockpit. The dc10's I've flown on had one in the cockpit and either 2 or 4 against the bulkhead facing backward. [Think Tom Hanks in Castaway. That was very accurate, you're staring at the first set of cargo containers behind a safety net the whole flight.] In the 747-100 there were about 10 airline seats in the upper deck and two bunks at the back. The 747-200's were the nicest. They were formerly passenger planes so the upper deck still had the first class seating.