Incorporating a contacts list from Numbers into Apple Contacts

This could be really simple, or it could be really complicated, depending on the number of contacts and changes.

At a high level, you can save your Numbers sheet as a .csv file, then import that into Contacts (via File -> Import)

Numbers will read the file and show an import map of its internal fields which you can then map to the import data (in case the fields are out of order, have different headers, etc.

Numbers should import any new contact (based, I believe, on matching names), and for any existing contacts it will prompt you to choose whether to keep the old contact data, replace the contact with the CSV data, merge the existing record with the new data, or create a new contact.

This can work well if your fields are consistent and similar (for example, if you built your spreadsheet from a Contacts export then the chances are there's a high degree of matching), but if there are many changes, or inconsistent data, this can be a challenge (although there is a 'Apply to All' option if you trust your data, so at least you don't have to click through each one).

The real key is matching the fields in your spreadsheet to what Contacts expects/understands.

if that approach doesn't work then you could import the spreadsheet data as new contacts and use Contacts' Card -> Look for Duplicates option to clean up the data. Again, viability depends on how many contacts you're dealing with.

If all else fails, it may be possible to write an AppleScript to perform more complex lookups/matching to update the data, but this will be very instance-specific so you're not likely to find an off-the-shelf solution.