COUNT(Fields!FieldName.Value) : Gives you the count of value in the current scope.
COUNT(Fields!FieldName.Value,"yourDataSetName") : Gives you the count of value in the dataset.
COUNT(Fields!FieldName.Value,"yourGroupName") : Gives you the count of value in the current Group.
You might have to return the desired value with your query if you are using filtering. Filtered values are not included in aggregates within groups. However, please test this at the dataset level by doing the following:
Place a text box above the table or matrix and give it a value of COUNT(Fields!FieldName.Value,"yourDataSetName"). If it does not equate then you will need to return this with your data.
Solution two
Open up Report Properties Code and add this custom code:
Public Shared Dim _RenderedCount As Integer=0
Public Shared Function ReInitRenderedCount() As string
_RenderedCount=0
return ""
End Function
Public Shared Function UpdateRenderedCount() As string
_RenderedCount = _RenderedCount +1
return ""
End Function
Public Shared Function GetRenderedCount() As Int
Dim ret As Int
ret=_RenderedCount
Return ret
End Function
I have added snippets like this before where basically you set a visible column with no background somewhere on your detail line, perhaps the last column. Then you can set its value=Code.UpdateRenderedCount() which updates the rendered count but returns "". You can then inspect and print the rendered count using label.value=Code.GetRenderedCount().