[UBL-SR-18]-Buyer VAT identifier shall occur maximum once
UBL-SR-18pxs-taxdata:TaxData/pxs-taxdata:ReportedTransaction/pxs-taxdata:ReportedDocument(count(cac:AccountingCustomerParty/cac:Party/cac:PartyTaxScheme[cac:TaxScheme/upper-case(cbc:ID)='VAT']/cbc:CompanyID) <= 1)