There is no limit on the number of friends you refer to Plutux Exchange, however the maximum amount of referral head count for Shard reward is counted as 20.
e.g. Alvin has referred 25 friends to register an account under his referral code, while all of them have not conpleted their identity verification. He will receive 40 Shards (2x20) instead of 50, but he can still refer more friends as well.
Check here for further on the rewards structure of our referral program.