Screenshot: Jake Peterson
For some reason, Apple only allows three fingerprints to be stored on your Mac, as opposed to the five entries allowed on Touch ID iPhones. While there is no way to add more custom fingerprints to your account, there is a way to double the number of fingerprints required to unlock your device.
When scanning your fingerprint from System settings> Touch ID, use one finger until you see “Keep collecting the edges of your fingerprint”. Now switch to another finger and complete the scan. It seems silly, but that a scan will now recognize both fingerprints. If you use this method for all three entries, your Mac will recognize six different fingerprints.