The application which we are referring to is the one created on this API console page.
Each instance you create in the above page is each App respectively. Let's assume you have created 2 applications. "AppOne and AppTwo".
You can make 2 API calls at the same time using AppOne and 20 using AppTwo.
If you feel that they might reach the concurrent limits, kindly create two apps as above and split the API calls between the two apps. In this case, if they make 30 API calls at a time, 20 must be made from AppOne and 10 from AppTwo, and so on. Once you deploy your developed application and monitor the usage of calls, you can use this strategy by not reaching the concurrent limits.
Again, I'm here to inform you that it will be 20/app from the Zoho API Console. If you create 3 applications in the API Console (https://api-console.zoho.com/), the limit will be 20 for each. I request you to kindly refer the below-mentioned article for more insights
https://www.zoho.com/crm/developer/docs/api/v3/api-limits.html#conc