I think your not getting many answers to your question because the no. of agents depends on a LOT of factors. Primarily it would be dependant upon the amount of business you can generate. In real terms it is also dependant on ones financial stregnth as it usually takes at least several months for the dollar inflow from first point of contact with a company. An example is a contact center in Bangalore when opened had 200 staff coming in daily for 6 months with no work! (and they are now one of the most repected in India).
So it boils down to these 2 issues – how much work you can generate (which in turn is dependant on the type of customers you are targetting, your marketing setup, your credentials, your references etc.) and financial viability.
However of interest to you would be the fact that from experience I have learned 80% of American companies would not even consider outsourcing to a contact center with less than 300 agents (americans love size remember!) This preference is not so strong in Europe. One prime reason for size is that this has now turned into an economies of scale operation. Technology investment in India is high and in order to leverage this investment fully from the lower labour costs, one needs to employ more knowledge workers.
Another practical reason why size is important is because most companies will initially only give a pilot project of say 25 seats. After satisfactory performance for 3 months they will expect you to ramp up to 150 seats for them (all such terms will be agreed priorhand in your SLA). So if one has 100 seats going up to 250 in one month can be an issue (unless you agree to pass overflow to another contact center but is your client agreeable to this?) but if one has 300 then going up to 450 is much easier.
Hope that was helpful. In a nutshell it says one needs a lot more info before they can answer your question. 🙂