How to get specific element Count in XML or XElement variable

XmlDocument xmldoc = new XmlDocument();
 xmldoc.Load(XmlPath);
var totalItems = xmldoc.SelectNodes(
         "/root/node/LastName/").Count;

Prerequisite: in order to use .Count() you need to import the namespace System.Linq:

using System.Linq;

You can filter the descendant elements using the Descendants method with the name "ID", then count the results:

int count = xml.Descendants("ID").Count();

Be aware that Descendants looks through all levels. If you had an element other than Person that also had an ID child element, you would want to be more specific. In that case, to count ID child elements that belong to Person elements, you would use:

int count = xml.Elements("Person")
               .Elements("ID")
               .Count();