In Helix, there doesn’t appear to be a way to easily get a games viewer count, currently you would have to page through the streams endpoint https://dev.twitch.tv/docs/api/reference/#get-streams using the game_id param to limit it to that specific game, and sum the viewer count from each stream object returned.
Alternatively there’s always v5 which has an endpoint exactly for what you want to accomplish https://dev.twitch.tv/docs/v5/reference/games/