@Meraki888 the API you reference is part of Wireless Health
https://documentation.meraki.com/MR/Wireless_Health
It's not going to tell you the number of clients connected to an AP, it's going to tell you how many failures for various tests have happened over a period of time.
I mean, if that's what you are really after then by all means, use whichever API call gives you the data you want.
The call I suggested will tell you how many clients are connected to a give AP, for the time period specified. It won't give you a count though... It'll give you a blob of JSON with details for each client, you'd then have to count that yourself in the script you're running to hit the API.
For the parameters, using them depends on the method you're using to interact with the API. Personally, I use Python, but to build on the curl example in the docs it'd be like this:
curl -L -H 'X-Cisco-Meraki-API-Key: <key>' -H 'Content-Type: application/json' -X GET 'https://api.meraki.com/api/v0/devices/{serial}/clients?t0=1579274691'
There the value for t0 is a time and date in Epoch time https://en.wikipedia.org/wiki/Unix_time
And timespan is just the number of seconds you want to report on.