Do settlers need ammunition?

[Settlers] don't consume ammunition provided they have at least one unit of the correct ammunition for their weapon. The same holds true with grenades. The only exceptions are the missile launcher or Fat Man, as these are tagged with "NPCs Consume Ammo".

Source: Nukapedia, "Settler" article

Try giving a settler a minigun and a 5mm round. =)

I belive that if you give a settler a gun with 1+ ammunition it will be unlimited. I have personally tested. Hope this helps :)

I tested this by giving some of my settlers one ammo only then attacking them. they were able to shoot me as much as they wanted without running out