how many of these converters do i need. whats with this rear and front rca plugs if the empeg only has one set of inputs. how is that handled?

The front and rear outputs of most car stereos are the same. You only need one pair. Just use either the front output or the rear output.

My opinion, though:

Dump the existing stereo and install ONLY the empeg. You will not miss the existing stereo, TRUST ME. It will simplify the installation immensely, reduce the chances for a ground loop, etc.