In your example, you would like to gate with !KeyDown (by voice) but count the number of times any key goes down. It suggests that you really want to differentiate between when any key goes down and when this specific voice's key goes down.
One way to do that would be using a TransformEventValues to "rewrite" any !KeyDowns that occur outside the context of the MIDIVoice and change them to an EventValue called !AnyKeyDown. Here's an AnyKeyDown example.
Alternatively, you could use ?VoiceNumber - 1 as the Index (see below)