As many as you like. There's no cap on the number of friends you can refer, and no cap on the rewards you can earn from them. Every referral who opens a card and starts spending earns you rewards, and the more active referrals you have, the higher your tier and the more you earn per referral.
Share your link with friends, communities, or your audience — the program is built to reward people who bring others in.
The only rules: referrals must be real people opening their own accounts, and each person can only be referred once. Fake, duplicate, or self-referred accounts don't qualify, and abuse of the program can result in rewards being withheld. See the referral terms [Sykka Referral Program Rules] for details.