Not allowing me to use >500 Vusers. Suggestion please

1. If I use 500 Vusers it is working, wherease if I use 700 vusers I am getting the following error message even though I selected more than one LG.

Error msg:
Could not reserve timeslot

Concurrent Project Vusers Limit exceeded

You need to decrease number of Vusers in Load Test configuration page.