Members may have up to 10,000 friends. Non-members may have a maximum of 500 friends.
Any unanswered friend requests you've received will count towards the total.
So, if you have 40 pending friend requests on your tree, you'll not be able to receive 40 new requests until you've processed the outstanding 40 on your tree.
For safety, never add people you do not know to your friends tree. Always ask your parents before adding a friend.