Well, as the others have alluded to, it depends.
Typically, with a known and trusted partner, you can go smaller by condensing gear. One stove for two people, for example. You save the weight and volume of an extra stove, pot, and wind-screen. Another example is sleeping bags, preferably the kinds that zip together. Two people need two bags. Two that zip together will allow +10-+15F in warmth, dropping weight and volume. Solo is another problem. You need to carry a bag for the full expected temperture, and a stove & fuel...you cannot share.
In both cases, Solo and Partnered, you will still be UL, but the volume will be different.
What type of shelter? Tarp? Tent? With a cooking area (bad weather expected?) Cowboy camping with a bivy? Each will have a differnt volume and weight.
IFF you are buying your first UL pack, but have hiking/camping expeience, likely a 3000ci pack will do. And something that will carry a 30# load comfortably. For UL equipment, plan on NO MORE than 20oz for a pack. The GG Gorilla for example fits these numbers. You can plan on 20# of food, and 10#base. This is about 10 days out.
There is nothing that says you *must* use that heavy of a pack. The Kumo or Murmur are excellent small volume packs. I use a Murmur for as long as two weeks out.
There are lots of other packs. But, to acheive a UL weight, you have to consider 20oz a max. weight. Your base weight needs to be less than ten pounds. For me, this rarely changes for three season camping. I vary slightly between 8-11 pounds, including camera, fishing gear, etc. I also have three light packs of differing sizes 2200, 3200 and 4000 cubic inches.