At the Parkmobile app you can add multiple number plates to an account, or just change the license number in the app.
See here the Parkmobile helpdesk for adding license plates: Parkmobile helpdesk adding license plates
And changing: Parkmobile how can I change my license plate
If you want to make a reservation at ParkBee for several license plates at the same time, you need per license plate a separate mail address.
It is not possible yet to make a reservation for multiple license plate with one mail address.