public class Program
    {
        public static void Main(string[] args)
        {
            string x = @"
<xml>
<id>123123</id>
<items>
    <model>
        <name>1</name>
        <age>2</age>
    </model>
<model>
        <name>3</name>
        <age>4</age>
    </model>
</items>
</xml>
";
            System.Xml.Serialization.XmlSerializer xs = new System.Xml.Serialization.XmlSerializer(typeof(A));
            var mmi = xs.Deserialize(new XmlTextReader(new StringReader(x))) as A;
            var list = mmi.list.Count;
        }

        [XmlRoot(ElementName = "xml")]
        public class A
        {
            [XmlElement("id")]
            public int id { get; set; }
            [XmlElement("items")]
            public List<B> list { get; set; }
        }
        [XmlRoot(ElementName = "model")]
        public class B
        {
            [XmlElement("name")]
            public string name { get; set; }
            [XmlElement("age")]
            public int age { get; set; }
        }
    }