Also another solution which I did consider suggesting but I think it might become a problem with a full server since the slender can't really kill all those people that quickly and it's really easy for them to scatter and collect them all so idk. Could always just make the required pages equal to a certain percentage of players on the server or something though.
Don't know if you've played as Slender before,but he can see everyone through the walls and how many pages they have collected so he would be able to set his priorities on the right people and kill them first,then get the rest.I however agree with having every page collected by players to count as everyone's so the game doesn't take as long.
Also,even though the pages collected by people will still count as one,Slendy could still see who has collected how many pages and each time he kills someone who has X amount of pages,the amount of pages that player had would be erased from the total score.