working.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Xml.Serialization;

static class P
{
    static void Main()
    {
        var xml = @"<ABRPayloadSearchResults xmlns:xsd=""w3.org/2001/XMLSchema"" xmlns:xsi=""w3.org/2001/XMLSchema-instance"" xmlns=""abr.business.gov.au/ABRXMLSearch""> <response> <abnList> <numberOfRecords>2680</numberOfRecords> <abn>95437472757</abn> <abn>38023383928</abn> <abn>12839919648</abn> <abn>50165055391</abn> <abn>60306167177</abn> </abnList> </response> </ABRPayloadSearchResults>";

        var ser = new XmlSerializer(typeof(Root));
        var root = (Root)ser.Deserialize(new StringReader(xml));
        var response = root.Response;
        var carList = response.AbnList;
        Console.WriteLine(carList.NumberOfRecords);
        if (carList.Items is not null)
        {
            foreach (var abn in carList.Items)
            {
                Console.WriteLine(abn);
            }
        }

    }
}

[XmlRoot("ABRPayloadSearchResults", Namespace = "abr.business.gov.au/ABRXMLSearch")]
public class Root
{
    [XmlElement("response")]
    public Response Response { get; set; }
}

public class Response
{
    [XmlElement("abnList")]
    public AbnList? AbnList { get; set; }
}

public class AbnList
{
    [XmlElement("numberOfRecords")]
    public int? NumberOfRecords { get; set; }

    [XmlElement("abn")]
    public List<string>? Items { get; set; }
}