I want more belt slots because I have so many mods and I want more things on my belt
how do I get more without making a datapack

without making a datapack? Probably make a custom mod that adds more slots or creates an item that gives you loads of new slots by wearing it

There is a configuration feature to adjust the amount of belt slots you get discussed here but if you want adjust what accessories go into what slot you will need to adjust the tags most likely for the given slot