I'm only doing this because it's the 500th post.

One for the ladies, (Edge, the ladies man)
Two for the man. (Palom and Porom helping Cecil)
Three you fight but once, (Magus Sisters)
Four you fight again. (Four Fiends)
Five once and only, (Only game with five people in party)
Six the most you'll see. (The most enemies you'll fight in a battle)
Seven means nothing,
But eight are the key. (Eight crystals to get to the Moon)

If your arithmetic is sharp,
Then soon enough you'll see,
This property of their relationship
Is the answer you must give me. (Give me the name of the game they're all from - FFIV)

Grover was closest, so he goes.