Professional guides will often do day-by-day plans outlining what you expect to accomplish on each day and where you expect to be. Each day is the associated with a given amount of supplies (mainly food).

Just go through it, add it all up, and pack it!

Last time I did a reasonable serious trip I was in the Drakensburg mountains in South Africa. I just figured out where I was hopefully going to stay each night, and then laid out my food on the floor of the hotel to figure out what I needed each day. I also planned for a shorter route I could take to leave early.

In the end I wasn't feeling so great and ate a lot less food than I brought... Fortunately I'm fat, so I've got plenty extra. 😂 (Seriously, pretty much everyone who isn't a cancer patient has at least a week or two worth of calories stored as fat!)

I also ended up using my backup plan, as I lost a day due to being unable to find enough water in one spot. So I spent a night somewhere I wasn't intending to that did have water, and made up for it by hiking out a shorter route with one less night spent somewhere else.