OK, found my answer !! instead of selecting a list I had to select only one node and use it's COUNT
XmlNode _Node = _Doc.SelectSingleNode("/VolSurface/MTM/Maturity");
int _CNode = _Node.Attributes.Count;
in this instance I was taking ALL the node available ( it was about 150+ nodes ) and for each node I was looping for attributes
XmlNodeList xn = _Doc.SelectNodes("/VolSurface/MTM/Maturity");
Any way , THANKS FOR THE HELP :)