My guess:
Person A refers a friend with their link
Person B activates the SIM they have
Person B/C activates (what sounds like) the BONUS SIM - but using the LINK
If in both cases the same referral link was used during activation then it would be 2 referrals. If the link was only used for the 1st SIM ONLY, then it counts as 1 referral. As JJP2 mentions better to check with the reps how it works in your case.
I assume that each 'activation' following the link counts as a referral but someone can correct me if not!
By 'following the link' i mean placing an order for the plan online, or activating an existing SIM already bought.