In common law, the “posse comitatus” was basically indistinguishable from our Consitutional concept of the “militia”, at the county level. By default, it consisted of all the armed, able-bodied men in the county.
Now, in practice, you are right, Sheriffs usually just deputized “picked men” rather than calling up the entire force of the posse, but by definition, the posse comitatus does include everyone that can get up and walk and shoulder a rifle.
But in practice, if the sheriff does as he says, he’s a freaking moron and he’ll turn his county into a war zone.