Can I sync more than thousand existing clients and invoices manually?
 

Yes, You can sync more than thousand of invoices and clients and payments at once using batch process concept. At WHMCS Smarters, Our module brings the ability to sync data manually and automatically.

Automatically : When an action has happened it WHMCS it triggers the same action to Xero Account at the same time. As soon as Client or Invoice is created / updated into WHMCS, it gets updated into Xero Account at the real time on the base of Data Mapping.
Manually : The module is also for you if you would like to sync your existing data from WHMCS to Xero.
Even, When a WHMCS creates clients and services and invoice as well itself by Cron , it triggers the same action to Xero Account.