//Define your data as list of maps as shown below
def data = [ 
             [number: 26354, price: 15000, date: '17-12-2017', customer: 'Clinton'],
	     [number: 16514, price: 28000, date: '24-08-2017', customer: 'Mark']
           ]

def xml = new groovy.xml.StreamingMarkupBuilder().bind {
    //Change the root element as needed instead of invoiceRequest
    invoiceRequest {
	invoices {
           //Loop thru list and create invoice elements
           data.each { inv ->
              invoice (number: inv.number) {
	         price (inv.price as double)
                 date (inv.date)
                 customer(inv.customer)
              }
           }
        }
    }
}
println groovy.xml.XmlUtil.serialize(xml)