Initially, you can add up to 20 friends and 20 enemies. You need to extend friends or enemy list if you reach the upper limit.
At the moment, you're allowed to have up to 100 friends and enemies. Every additional friend or enemy added will cost you 1 gold. You can choose the number of friends/enemies to add. The minimum number is 1. Cost will also be shown to you. Once you confirm the number, gold will be deducted.
Extend Friends List and Enemy List are independent to each other.