I just figured this out. There is a way to have more than 3 DEAD spiders on the map. I started a game and built two mono's. I then paused the game and built two spiders at the exact same time. When they both were done building, they were created from the same body. When those two (and the third one I built) died, I still had a 4th spider body in my base. I did it several times and ended up with 7 dead spiders on the map. I can post a demo if someone likes, but while you can make a lot of dead spider bodies, you still can't have more than three. I can see how this would come up in an AI match, as building units at the exact same time is probably very common with AI.
Here are a few pictures to illustrate the point.
This shows me doing this method a couple times and creating several extra dead spider bodies
This shows me building three at the same time with 3 monoliths. All three res from the same body.
Just to show that the body count is about to go up by 2, here is another picture of them spread out.