After trying this card a bit i'm actually very impressed.
This is a hybrid sort of card, it's a Tutor that actually searches for cards in bulk. That makes Backpack a weird sort of cross between Preposterous Sketches and Prepared for the Worst. So, what exactly does it do?
First off, this is a tutor effect. If you have powerful items or supply cards like Lightning Gun, Key of Ys, Strange Solution or The Gold Pocket Watch then this thing greatly hastens the rate at which you get them in play. This effect can be absolutely worth it even if you have only a moderate number of targets in your deck overall.
Second, this is a "bulk" tutor, it doesn't just hit some small and limited pool of key cards, it hits a variety of helpful cards like Emergency Cache, Knife and Flashlight, this has the double effect of A: drawing cards and thus shrinking your deck. B: Getting chaff and gunk out of your deck, thus shrinking it. The latter effect is actually better than it sounds, often you're looking to draw an important event or asset (Rite of Seeking for example or "I've got a plan!") and getting all your Flashlights and Emergency Caches out of the way can be invaluable, especially since these cards don't occupy hand space and thus don't mess with you when you're drawing aggressively.
Third, if you're looking for particular cards (as is usually the case when you slot tutors), even if you don't hit all three cards it is still probably a decent play. If you hit just one card, if that card is something like Lightning Gun, then it was still probably a fine play. If you don't draw the thing you're looking for, or nothing at all! then it was still probably a good play because at least the deck is shuffled, thus your chance of drawing the thing you need quickly is refreshed. In other words: Either you get what you wanted or you get a "re-roll" on the odds of drawing it soon.
So, how many items/supplies do you need to warrant backpack?
I've tried some item heavy dudes like William Yorick and folks with just normal item amounts. Generally I lend higher priority to backpack when there's upgraded targets in the mix. As a rule I found that hitting just 2 targets is perfectly acceptable.
8-9 viable targets is the absolute minimum and I'dd only recommend it if there's cards in there like Key of Ys, Strange Solution or The Gold Pocket Watch that are really powerful and can't be tutored with Prepared for the Worst for some reason.