This is the API I call to get client count per AP
https://api.meraki.com/api/v1/networks/<NetworkID>/wireless/clientCountHistory?timespan=600&resoluti...
I pass the network ID, the band either 2.4 or 5 Ghz abd serial number of the AP, the time span I use is 10 minutes and this API is called every 10 minutes.
Once the results are returned back as a json file, I then do the processing to extract the details and write to a back end SQL database. I then have a Power BI report that can graph the AP client count over a period of time.
I did a very quick and dirty way to get this information am sure there is a better way but dont have time to look into this. For my purpose, the above works for me and been running past 12 months
More info here
https://developer.cisco.com/meraki/api-latest/#!get-network-wireless-client-count-history