I am doing a modpack for me and my friends to play, since am adding few addons for apotheosis and irons and spell books i need a lot of slots, i wanted to add 32 but it limits me to 16 from what i seen. Is there a way i can change the limitation? thanks.

No, the 16 slot limitation is hardcoded. I have no plans to change it.

Even 16 slots tends to overload the tooltip space significantly.