Hey @Christian8
You can query the graph to obtain this data, albeit this a slightly more complex approach as you'll need to manage authentication, HTTP request / response, etc. - see https://docs.microsoft.com/en-us/graph/api/reportroot-getemailactivityusercounts?view=graph-rest-1.0&tabs=http
Your current Flow isn't to far away from recording email activity, but you need to make a decision on how you wish to record the data as this will affect how you store the data in SharePoint... if it's simply a total count then you can just add a single list item, and increment every time an email is received... if you need a daily, weekly, monthly count etc. then you'll need to create a list item for every mail received, you can then use the 'SharePoint Get Items' action and use the 'Filter Query' to return the items which meet the time-frame criteria (1 day, week, etc)... finally just check the length of the returned array.
Finally, I don't think you can record emails sent in this way as there isn't a Flow trigger for sent email.
I'd suggest querying the graph is probably the best solution.
HTH
Jay