You need more for a manned mission because of the harmony requirements when deployed - ie. if you can only deploy for 6 months every 2 years then you need to have a pool of aircrew 4 times the size. Furthermore, depending on the mission that you are doing then unmanned tends to have a longer flight time. If you are doing CAS with a Reaper that can fly non-stop for 14hrs doing overwatch/CAS then you will only need 3 airframes (2 + 1 spare to accomplish it). If you do the same thing with Tornado GR4 then you will need at least 8 airframes, a series of tankers and a huge supply of spares! But then you don't get a supersonic reactive capability that can operate in a significantly contested environment if you use Reaper.
As ever, the choice requires comprimises as all things aviation ever did. But in answer to your first question, there are manpower savings to be had when deployed for ops.
LJ
