Good place to start!
You dont need to worry about getting numbers yourself. The organisors will produce a seeded entry list and your number will be based on that. For a National B event you will be given Rally Plates at the start of the event to put on your rear nearside window and back window. If its just a club 12-car you probably wont get anything.
IIRC the white squares you put on your doors are not permitted in road events, and neither is advertising on the car, so make sure you dont have any of that or you wont get through scrutineering.