You can try doing what Standard GP does when it fetch PO Number or SOP Document Numers, though it would require a bit more work.
Save the "Next Customer Number" in a Custom Table (or you can even check some of the RM Setup User-defined fields or some place where you can manage to find in GP itself).
While entering a new Customer, fetch the value from this field and Increment it immediately. So even if multiple users try to create a Customer, the Increment will take care, though fractions of time does matter and sometimes you tend to get a clash, but that's rare.
I guess, that's one option you can try out.
Thanks
Vaidy
http://vmdyngp.blogspot.com