Reading the API docs, Under 'All Models' you have listed 'ActivityTotal’ with a ‘count’ parameter
https://developers.strava.com/docs/reference/#api-models-ActivityTotal
While I know I can do this by calling https://www.strava.com/api/v3/athlete/activities. The problem there is that I would have to paginate to get all the responses which in my case takes a long time.
So if there is a way to just get the total number of activities in the account by a count of ActivityTotal then that would be a big help.