I remember when I used to goto Bob's single player maps in geocities, probabey my favorite website but got taken down when Yahoo canceled geocitites. Also I am pretty sure that Mikko's site and Duke4 don't have even 5,000 maps but if they do I plan to get many maps... MANY MAPS MUAHAHAHAHAHAHAHAHA!!
This site has over 600. If I had to guess MSDN probably has double that (but alot of them are going to be the same).
Duke4 has a few and that crappy 4000 map pak (which is only really 2or 3 thousand because most of them are doubles).
Poda's site, I would guess, has around 500, but none of them come with text files. If you google Duke Nukem maps you'll probably find alot of those dead sites that are still around, like Bob's which is still there, but hasn't been updated in years.