User blog comment:Zack Tan/Volume 7 Intermission/@comment-26836039-20150905060251/@comment-26951739-20150905095632

It´s cleary said that you needed a minimum of 41 Members to build a guild base.

Ainz further explained that there was another guild that infiltrated other guilds to gather information. Because of that, Ainz Ooal Gown never took in more members as the needed 41 players. If infiltration would not be that dangerous, he claimed they probably would have had more members.

I would just copy+paste the text, but have no idea when he talked about it. So searching for it is quite time consuming for me, and there are just new translations for volume 6, so i´ll be a bit busy the next hour. :)