With the JPAC you get the JAMMA inputs of 1 stick per player, 3 buttons per player, 2 start buttons and 2 coin inputs. The JPAC itself has screw terminals allowing for 4,5,6,7 and 8 for both players. So theoretically 1 stick, 8 buttons, coin and start PER player.
You can use an IPAC with a JPAC if you need additional inputs but that is typically used for 4 player set ups.
The JPACs primary purpose is to use a JAMMA cab's existing wiring. You would still need to supply the JPAC with a valid video signal to display on the arcade monitor (groovyMAME, Soft15, ArcadeVGA,etc)
You cannot use the guns from lethal enforcers on MAME, they dont just plug into a PC and work like that.