I think what you're looking for is Organization > Summary report. Select Entire Organization, under NETWORK(S) then the time frame you want, next to Summary Report, right at the top - then look at the Client stats on the RHS. You can export to Excel if you wish, top right.
If you need more granular than that, I think you'll need to use the API.
If you don't want all Networks, but still more than one, you will need to add a Tag to all the required ones, and filter by that
