Uhhhh......you do remember what website you're on right? Silly man. Asking silly questions like "do I need more than one".
My last count was 6 of them (all different of course). 5 Armalite's and 1 Black Rain (that I still haven't assembled yet).
Think of it as options. When they're built for specific purposes, they tend to do the job better. You can go from shorter ones designed and built for kicking in doors and clearing, to longer ones designed to plant on a hilltop and take out targets on the opposing hilltop. It's all in what you build them for, and what you expect to do with them.