Hello @Greeshma_Sampath1
I beileve it is really difficult to get to a number only for web app call because the the CPU and RAM are shared resources among different modules.
Generally the hardware sizing is done on the basis of caching,storage and the email sending volume.
You can reach out to Adobe Professional services team for some recommendations.
If you need more information on Hardware Sizing, you can refer to this page: https://helpx.adobe.com/in/campaign/kb/hardware-sizing-guide.html