Hi, jessieTS
Thanks for your post, and welcome to the MYOB Community Forum.
We truly appreciate gavin12345 for the unending support in the MYOB Community Forum.
Regarding your concern, sending an update event ensures that the ATO will receive the most up-to-date figures for your employees. When you mention a trial run, are you going to process a pay run for the employee? Also, when updating the pay history of each employee, kindly go to the Card file > Cards List and then select the employee name > Click Payroll Details tab > Pay History. This window will allow you to zero out the pay history, but kindly make sure that you have a copy of the pay history. Lastly, there is no maximum number of employees that you can record in payroll. You may also check The Help Article: End of year finalisation with Single Touch Payroll reporting for further information when sending an update event.
Please let us know if you require any further assistance with your concern.
Best regards,
Doreen