Yes, it's a very common tactic to pack at least two launchers for constant coverage.
Same as Kerrash.
I have managed to make NPCs run out of SCBs, chaff, missiles and rail gun ammo (all of that takes a long time - you are more likely to win, lose or flee).
The only thing they are officially supposed to have infinite ammo of are multi cannons, however similar to players I suspect they currently also have infinite cannon and plasma accelerator ammo.
Additionally elite NPCs may have modified modules (only modified weapons were removed from them post 2.1). Which can give them higher wep capacity/recharge and other things.
Also, while they do not seem to care about their heat levels at all (firing lots of weapons and banks without heat sinks), their modules do take heat damage (check all sub targets - the more they have the more it gets spread out).
They usually fail to provide proof. (apart from someone recently correctly counting too many cannon shots after rail guns ran out - which is true for players as well right now)
Just because a huge stack feels infinite to you doesn't mean it is if you don't count it. And even then all you can manage is discovering that they used more than a normal clip should have, which still technically isn't infinite