Why not contact their authors and see how they've managed it?Good idea.
//EDIT:
I tried to reduce the map's entity amount to 1024 and it didn't run. After deleting about 40 more I finally managed to run it! Thanks guys!
Unfortunately, I'm now having problems with huge epoly counts. Sometimes it gets even up to 17 000 and the game is really laggy. I'm going to delete some monsters and make some of them to spawn a little bit later in the map so there wont be so many monsters living at the same time. I'm not sure if it helps tho.
I'm also having a small problem with one texture. When playing the map it seems to be another one. The texture is -0C2A4_PAN2 and it gets replaced by -3C2A4_PAN2. They are both from halflife.wad and are very similiar. Does anyone know how this can be fixed?
////EDIT:
Fixed the texture problem. I used another texture.
I found out that the actual cause of the error "item count is over the 512 limit" was one sound replacement file, not the entities. I was using too many sound files. Deleting only soundfiles didn't help, I had to change the sound replacement file!
Why I first thought it was about the entities was because when i deleted some more entities I started to save the map with a different name and I didn't have the sound replacement files in this new map's sound folder. Because I didn't have them there, the map worked and the only thing I could think about was the entities.
Sorry about the mess.