If you’re providing your own food for the trip you need to understand how many meals you might expect to bring.
For McCarthy based trips you won’t eat any meals in the backcountry until lunchtime on Day 2.
A trip billed as a 12 day trip, for example, would require you to pack 10 days worth of food plus extra food in case weather delays our pickup. It’s always wise to allow an extra day of food (or at least some form of snacks and food for an extra day). All Expeditions Alaska trips to Wrangell – St. Elias nNational Park, unless specifically scheduled otherwise, will include a day travel to get to McCarthy, a day to return to Anchorage, and we fly out of the backcountry usually in the morning of the 2nd last day.
So for a trip scheduled, for example, Aug 1-12, you’ll need to backpack
lunch and dinner on Aug 2
breakfast, lunch and dinner Aug 3-10
breakfast and maybe lunch of Aug 11
So I would suggest at least 10 full days food for your backpack.