First of all, find in your mmo.h:
#define MAX_GUILD 16+10*6
16 is the primary value, it means, it's how many members a Guild can have at Level 0 of Guild Expansion Skill. So, change it anyway you want, for example, for the 30 you said:
#define MAX_GUILD 10+10*2
It means, 10 at Level 0 and plus 2 per Skill Level. Once it's done, for the existing Guilds, you need to set some things in your SQL Database: Let me say first that, If I were you, I would choose to delete all Guilds an make an Event with Increased Guild Experience than do this... There are examples here, here, here, here, here, here and here.