Afaik flows are as defined here https://en.wikipedia.org/wiki/NetFlow so numFlows is how many flows. You must have traffic analysis enabled for this, and of course you need to specify the client of interest.
The API documentation doesn't say what period the client flow data covers, I'd guess the last rolling month, should be clear if you look at the result data.
For network-wide client info, I prefer https://developer.cisco.com/meraki/api-v1/#!get-network-clients which gets you details including data sent/received for every client on the network within the timespan.
You could start with that, then select the high usage clients (based on sent/received figures) for further analysis down to flows.