Hiring a company secretary to incorporate companies

Hi,

I own a private limited company that provides various consulting services to our clients.

I want to employ a CS who has the necessary certification to incorporate companies by filing and certifying all forms related to company incorporation on MCA portal. 

What certification should the CS have? Should he/she be a practising CS or a whole time CS? Is there an upper limit to the number of companies that can be incorporated in a year?

Thanks for clarifying.