I know this was asked 8 months ago but please, hear me out, youre all wrong:
37 slots in a player inventory. Lets say we use 36 for shulkerboxes and 1 for an enderchest to add another 27 slots.
so youve got 63 slots total to fill with Shulkerboxes. 63 * 27 (slots in a shulkerbox) = 1701 slots total.
1701 stacks of items! which is 1701 * 64 = 108,864 items total in the shulkerboxes and enderchest on you!
But wait theres more.. lets say if you were carrying Ingots.. you could transform the items into nuggets.. which means..... 108,864 * 9 = 979,776 Nuggets at once.
Now if you want the total ITEMS, you take that number, add the shulkerboxes and enderchest to it, and 4 armor slots (the shulkerboxes and enderchest and armour are items as well) which is a grande total of 979,776 + 27 + 37 + 4 = 979,844 items.
(which is btw 283.5 Double chests full of items! all on you at once!)