A light party is 4 players.
A full party is 8 players.
An alliance is 24 players.

They won't add an extra player because the party structure is built from a base of 4 players. Double a light party (2 tanks, 2 healers, 4 DPS) and you get a full party. Three full parties makes an alliance. Everything is modeled around the mechanics of a 4-man group. World of Warcraft was built the same way except that it was built around a 5-man group model and multiples of 5.