Exactly what the error says on the tin:
You have so many entities on your map that during run-time the game exceeded the 2048 edict limit.
You need to lower the number of entities on your map (e.g. prop_dynamic to name one) to < 1500 so that there are enough free edict slots for players, projectiles, etc (run-time entity creation)
You can type "status" on your console to get the edict usage at any point in time.