Try googling for Access Custom Autonumbers.

The method is to have two separate fields called, say, OwnerID and
DogNumber. You can find the next vacant dognumber by calling

 DMax("DogNumber", "Dogs", "OwnerID=27")

This will return the largest-so-far for this particular owner, or else a
Null if there are no dogs registered to him or her. You can add 1 to the
number in order to get the next one. You have restrict all data entry to a
form, because there is no way to get code to run if a dog is added using
VBA, Excel, querydef, etc.

I would be a little suspicious of the method, though. What happens when Reg
passes or sells his dog to Wendy - does the dog change from 23:2 to 17:12?
What happens to all the dependent tables with Foreign Key values pointed at
the old dog number? Still, you know your business rules!

All the best

Tim F