5 is fine i think but im sure they add later on GvG lets say 50v50 guild fight where the party of 5 ppl is kinda like a squad.
I like the idea 50 ppl in a guild and you have like 10 squads for example a healing squad or you mix them dps and support etc.
I think you get the idea XD
I’m fine with the current 5 max if they don’t change the exp range. It’d be too many people in an area if they all wanted to share exp. If they increase the range I would reconsider though.
This is assuming they’ll have some sort of raid system where you can link up parties though.
Nine will probably be max based on Centurion having 8 spots around them.
i have see in database we can be 10 after 
(after the rank 10 i suppose)