When I was reducing my underwear supply, I couldn't find any organization that would take it, even though some of it had hardly ever been worn.
Curious that you still have about half of your underwear supply if you wear it less than once a week. I would think 1 -3 pairs would be enough.
When I retired, I kept all of my underwear for about a year, and then reduced my supply of about 20 pair to 10. I soon realize that it was still more than I needed, and made further reductions to 6 and 3 pair shortly thereafter. Stayed at 3 for about 6 months and then decided to go full time except for doctor's appointments so got rid of 2 more. 4 months and 1 doctor appointment later, I disposed of the last one and am now underwear free.