And not all envoys generate additional tiles for a CS. Only envoys that establish a new “high water mark” for envoys at that CS give more tiles. So, for example, if you see a CS already has 7 envoys from another civ, and you can send envoys that would put you at 6 envoys with that CS, sending those envoys will not yield more tiles for that CS, since the high water mark (7 envoys) remains unchanged.