[SiC]

Accutally even though 18 is the max for an alliance you can call for help and other people will be able to join in the battle so you can have a renkei as big as you want (you may actually experience this in the level 55 cap lifting quest). I wont go into too much details about that quest but you'll have to fight 3 notorious monsters. If you dont have any people higher than level 55 to help you, you'll probably need more than 1 alliance to defeat them (thus you'll need to call for help).

Anyways in normal battles you probably will never have more than a 3 WS renkei. Even when fighting HNMs we usually just do a 3 or 4 WS renkei. The max I've had in a renkei was about 10 WSs and even at 10 it gets confusing so I wouldn't even want to try anything beyond that