Hey KurtDuane,
Thanks for using Apple Support Communities. As it turns out, you can have a total of 50,000 contact cards with iCloud. Those details and other iCloud limits can be found here: Limits for iCloud Contacts, Calendars, Reminders, Bookmarks, and Maps - Apple Support
Let's start to narrow this down to resolve it. As always, you'll want to be sure your Mac is fully updated as updates help by installing those bug fixes and security updates. Here's a guide on updating, just in case: Update macOS on Mac - Apple Support
Another good idea is to check to see which accounts are syncing with your contacts. Is it only iCloud? If you're using more than iCloud, turn the accounts off one at a time and test to see if the behavior continues in the Contacts app. Here's a guide that will help to check. You don't mention the macOS version installed, but it can be changed at the top of the guide: Add contacts from iCloud, Google, and more to Contacts on Mac - Apple Support
Let us know what you find!
Take care!