There is no limit. Currently you get 50 IP for each referral. I can confirm you've had three referrals, and also that your IP doesn't reflect that. Looks like there's a bug in the registration page. My guess is only people who enter friend codes in the profile page are being given credit.
Thanks for bringing it to our attention. We'll take a look and get it fixed.
Please note, the problem is in the reporting, not the recording. Feel free to continue to share your friend code and when we fix the issue, your IP will be accurately displayed.
Cheers!