Dynamics 365 for Finance supports using a shared number sequence for customers. This way, customers in one legal entity have the same customer number if they exist in another when created from the All customers list.

When creating a Customer from the All customers list page, and entering the name of a Customer that exists in a separate legal entity, the "Existing customer. Copy customer?" toggle box is visible, and can be turned on to copy the Customer and its Customer number from the other legal entity.

However, this function is NOT available when converting a Prospect to a Customer to see if any Customers associated with the same Global address book Party exist in other legal entities, and to reuse that Customer number. As a result, the functionality is incomplete and will lead to multiple customer numbers pointing to the same Customer.