Maximum $200 can be added and  bonus credit is only to first top up.

Looks like it's world wide.

https://www.msn.com/en-nz/news/techandscience/apple-is-giving-users-a-10-percent-when-they-add-money-to-their-account/ar-BBUBVb9

Edit: I didn't add funds but didn't look like it was showing extra on mine, I picked $50 and that's all that it showed as adding, so backed out of transaction.