How do Mercenaries work: placement, recruitment, standing alone and troop size?

Question

Accepted Answer

Buying the tile and placing the three Mercenaries in your city is not a recruitment, so Enforced Recruitment does not apply then; it does when you recruit them later, as they count as basic units. They can stand alone, give 1 PP each when recalled, and obey normal troop size limits.

Kemet

How do Mercenaries work: placement, recruitment, standing alone and troop size?