There’s no way to see this right now from our UI. If you really want to get a value and don’t mind working through the data, you can pull the graph data from the page source/http request. It’s going to be a json object that you can manually sum up.
For example:
You can request https://itch.io/game/summary/GAME_ID?group_by=daily&range_left=2019-12-01&range_right=2019-12-31 with accept ending application/json. (Or if you load it in your browser, the json object is in the source of the page)
It will return the daily stats for the month of December for GAME_ID. The format should be easy to parse and calculate a sum from.
Note that this is not a documented API, so I can’t guarantee it will be available at that URL or in that format forever, but you’re welcome to use it now to fetch the data.