How many fingerprints can you register to a single user on your Mac?

You can register up to three fingerprints per user account on your Mac, with a total of five fingerprints across accounts. To add or delete fingerprints, you need to go to Settings > General > Profile & Device Management and click on Add Fingerprint or Delete Fingerprint. You can also choose which options you want to use with Touch ID, such as unlocking your Mac, making purchases, or using Apple Pay. References:

How to use Touch ID on MacBook Air or MacBook Pro | iMore

Add or delete fingerprints on your Mac - Apple Support

How To Set Up Fingerprints on Your MacBook and Use Touch ID