Referrals that qualify for credit are part of the Refer a Friend program that began December 26, 2018, which introduced a legal agreement for the referrer that allows for referral credits to the referrer and her referred friend. Any referrals you made before this date aren’t eligible for credit to you.
How many friends can you invite? All of them! But you will only be compensated up to 20 friends who fund their accounts in each calendar year.