In order to unlock this achivement, you need to play horde with 20 different teammates.
You can play with people in one clan, leave, and join another clan and your progress with still carry over. Horde Battle tokens are rare, and sometimes hard to get, so make sure each person that joins you is different. Otherwise, it will not count.
You do not need to win or complete horde battle for it to count, I lost many battles with new people, and it still counted for my progress.
If you reset your account for any reason, your progress will carry over. However, it will not update until you have played a horde battle.
If you need a group, I am currently in the Camshee crew and we are active daily and can help you get a few unique hordes done.