Serial# by ID count

[freecolours]

I'm looking for a way to create a serial# in an orderdatabase, based upon costumerID and total of records from the same costumer.

Example: Company1 has costumerID 523, and has 3 orders (records in DB) booked. So serial# 1 = 5231, serial# 2 = 5232, serial# 3 = 5233, etc.

Company2 has costumerID 631, and has 5 orders (records in DB) booked. So serial# 1 = 6311, serial# 2 = 6312, serial# 3 = 6313, etc.

I guess it's something with a calculation, but I can't figure it out...