>>440353It depends a lot on your gear and how you intend to pack it.
Generally speaking, your pack will have a basic kit, plus food for the number of days you stay (around 1L pack volume per day). What your basic kit makes up can vary widely, and how you pack matters as well. The basics are then your sleeping kit, extra clothes, cookware, first aid, and 1-2L of water.
From my experience, I can get a warm-weather multiday trip into a 30L pack with some caveats: bivvy sack instead of tent, inflatable pad instead of foam, and no camp stove/fuel (maybe a firebox stove or similar would be fine since pack size is very compact compared to carrying fuel). It may be doable on shoulder seasons as well, but definitely not in cold weather.
It's also important with internal frame packs to consider what weight load they were designed for, as too much weight will deform the frame or not carry comfortably. You have to avoid swapping large bulky items for smaller but heavier ones. External frames are generally thick aluminium that is strong enough to handle basically whatever you would ever wants to carry.
IMO if you want to get into multiday and only get one pack, I would recommend just getting a large pack around 60-70L. having a little bit of deadspace on shorter/lighter trips is much more manageable than trying to figure out how to manage longer ones.
