A system admin used NPSP Contact Merge and notices the number of household accounts has...

https://powerofus.force.com/s/article/NPSP-Merging-Contacts

When using the NPSP Contact Merge functionality, any empty Household Accounts that do not have associated Contacts or Opportunities are automatically deleted. This is likely the cause of the reduction in the number of household accounts noticed by the system administrator.

Steps to Verify:

Check Merge Settings:

Review the settings and configuration of the NPSP Contact Merge process to understand its behavior regarding empty Household Accounts.

Review Audit Logs:

Check the Salesforce audit logs to see the details of the merge operations and confirm that empty Household Accounts were deleted.

Restore if Necessary:

If the deletion of empty Household Accounts was unintended, use data recovery options to restore any necessary records.