Sharpshooters or designated marksmen: riflemen trained and equipped to engage with precision rifle fire at distances past the effective range of the average troop with the issue rifle; however, not trained or equipped (or experienced enough?) for true sniper operations.

How many are needed and where do they belong?

One per rifle squad? In a platoon weapons squad? In a company weapons platoon to be to be employed at company level or attached out as needed?

At one time I would have said one per rifle squad but this assumed squad fire and maneuver. That would have allowed the squad's DM to stay with the squad's fire element. But in an understrength squad, a squad that probably has to fire or maneuver, a DM at squad level doesn't seem like a good idea.

I don't think you would want a DM at squad level if that squad has to conduct an envelopment, would you?