i have a 27 inch mac with 21000 contacts as i run a business. it says out of memory when i am trying to delete duplicates and the app crashes I have 10 Gb Ram Please help

21,000 contacts is fine, the app is more than capable of handling twice as many (Limits for iCloud Contacts, Calendars, Reminders, and Bookmarks - Apple Support), however you might want to check that you haven't exceeded some of the other limits associated with contacts, you might also want to check that you haven't introduced any illegal characters into any of the contact fields (especially the notes)

Do you mean you actually tried to put 21,000 entries in the Contacts app? I would think that app would error out at less than 10% of that! Contacts app is fine for a few hundred, but not many thousands of entries. There are much better specialized applications: so-called CRM Systems.

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Etrecheck – System Information

How much free disk do you have? How much free iCloud storage?

What OS are you using?

LN