Disk Utility First Aid warning in the otherwise okay results.

2b1 wrote:

...would you please direct me to where the safe Mode reboot procedure is found.

Use safe mode to isolate issues with your Mac - Apple Support

regarding my backups: what will I need to restore from my backups?

Whatever you have previously decided to use for backups. Time Machine, maybe Carbon Copy Cloner, whatever.

If you don’t currently have backups, boot Recovery and use Disk Utility, or maybe use CCC to create a backup onto external scratch storage.

All storage hardware inevitably and invariably fails, if it doesn’t get lost or stolen or corrupted first. Or flooded, blown away, or burned up in a fire. Sometimes failing with little or no warning. Backups are a way to reduce that exposure.

If the repair finished with an okay, I wouldn’t be fussed about this.

If you want to pursue and have add-on anti-malware, add-on cleaning apps, add-on network-monitoring-and-firewall apps, or similar tools around, I’d remove those per the vendors’ instructions, reboot, and test again.

Failing that, I’d ring up Apple Support for a discussion.

If you’re running consistent backups with some depth (and as you probably should), your data should be preserved either on the internal device or on the Time Machine backups device.

There’ve been a few other discussions... Disk Util Error Message apfs_num - Apple Community

Safe Boot from the HD, (holding Shift key down at bootup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.

You’ve run the tests. While in Safe Mode, the tests are fine, a d correct the error at least temporarily. While not booted in Safe Mode, the tests are reporting this diagnostic. But in either case, the tests do finish with an indication that the file system is okay, other than this count.

If you want to pursue and if you have have add-on anti-malware installed, or add-on cleaning apps, or add-on network-monitoring-and-firewall apps, or similar tools around, I’d remove those per the vendors’ instructions, reboot, and test again.

Failing that, I’d ring up Apple Support for a discussion.

If you’re running consistent backups with some depth (and as you probably should), your data should be preserved either on the internal device or on the Time Machine backups device.

You have complete and current backups, so you have a path to recovery.

Then reboot into Safe Mode, and try the Disk Utility tests again.