As many as we have open seats in the back. On the Delta side of the house it can be a little different. We'll take as many as the gate will send out, but our deal with Delta supposedly will not technically allow us to take more than one. Many stations will still send them all out, though.
Wish I could tell you there was a consistent policy.
Right now Delta and United have both told ACA that they are paying us "fee for departure" and therefore own the seats. Ergo, they will only allow one jumpseat per plane.
Our ALPA contract with ACA states that we have the opportunity to extend "unlimited" jumpseats - meaning if there is a seat open you are welcome to it.
Delta, United and ACA pilots are welcome to sit up front in the cockpit. All other 121 carrier pilots are restricted to the back if a seat is available.
A very important key to getting aboard is who is running the gate. If the gate agent is a United or Delta employee, forget about trying to get aboard if you are a second jumpseater - they will hold religiously to one seat, one rider rule. If the gate is being run by ACA, ask to speak to the captain about you riding along if seats are available. Most of our guys will welcome you aboard, if there's an open seat - the only caveat is that if the plane is nearly full, you may have to stand out in the rain until we get an official pax cut off from the gate agent.
Good luck. I'm a commuter, too. If there's a seat open and you need to get to work or home, you're coming with me.