Can the customer number be different from the CRM account number?

No, the customer number and the CRM account number are linked and must be the same. The CRM account serves as a kind of parent having several different possible children. Because of this linkage, a single business entity can be stored in xTuple as both a customer and a vendor. This helps to simply and cover the cases where you sell to one company as a customer, but you also buy from them as a vendor.