If the number of people in your Facebook custom audience doesn’t match the number of people in your synced Klaviyo list, there are two factors that could have caused the audience to have a smaller number of profiles than your list or segment in Klaviyo.
First, it can sometimes take Facebook up to 24 hours to process profiles synced from Klaviyo. If your custom audience does not have as many profiles as you expected, make sure that you've given Facebook 24 hours to fully process all of the profiles from your sync.
Second, it is likely that a percentage of the email addresses you attempt to sync will not match with an existing Facebook account. This is why your Facebook Custom Audience size may be smaller than the linked list or segment in Klaviyo. Facebook will not return an error, but won't add any contacts who have email addresses that don't match to an existing Facebook account.
One troubleshooting step you can take is to try syncing the same list in Klaviyo to a new custom audience by selecting + Add Custom Audience sync (under Custom Audience Settings) on your Facebook Advertising Integration settings page.
Once you've selected the list or segment and audience for your new sync, click Update Settings.