| Brand | Number |
|---|---|
| Visa | 4532759734545858 |
| MasterCard | 5409889944179029 |
| Discover | 6011033621379697 |
| Diners Club | 36004244846408 |
| JCB | 3566005734880650 |
| American Express | 371263462726550 |
| China UnionPay | 6222520119138184 |
https://docs.connect.squareup.com/articles/using-sandbox/#cardbrands
You can use any future expiration date and any CVV except 911 (which generates an error)
Unlike all the other payment services, Square does not allow to populate the payment form fields programmatically, so the Magento 2 Swuare extension does not contain the standard «Prefill the Payment Form with Test Data?» option, and you should to fill the payment form manually each time.