Single stat beginner question

Folks, I am trying to count on the amount of values that I am getting in the query (like indexes or string values), My query is from Graphite datasource
criteria1.criteria2.na.*

in query inspector I get this:
response:Array[11]
0:Object
leaf:0
context:Object
text:“cpu”
expandable:1
id:”metric1.metric2.metric3.*.cpu"

Basically what I would like to do is:
count(select metric1.metric2.metric3.*)

Yet my panel still shows N/A, any help would be really appreciated