Display # box

-- HikaShop version -- : 4.4.2
-- Joomla version -- : 3.9.26
-- PHP version -- : 7.3.28

I have been trying to find some information about this to no avail. What controls what numbers are in the Display # box? 21, 42, 63, etc. are strange numbers and I would prefer to change those out.

That number is generated automatically by the number of products that you set in your menu, when you set the listinga page for products.
In your case, you show 21 product on page#1, on page #2 you have another 21 product, so , 21+21 = 42, in page#3 you have another 21 product, so the number in the dropdown it will be 63.

I see. Thanks for the info.