Assume you are using this API call...
https://developer.cisco.com/meraki/api-v1/get-network-clients/
If you look at the details you will see it defaults to 10 clients. You need to set the perPage parameter to get more per page, for this call it can be up to 5000 clients per page.
As you are only getting 10, it sounds like you are not handling multi-page responses, you need to do this otherwise if the number of clients exceeds the perPage value you still will not get all clients.
For info on pagination see... https://developer.cisco.com/meraki/api-v1/pagination/#pagination