There is no concrete answer for this question as it will be based on a number of factors including a hosting company's total sales vs expenses including the cost to maintain equipment and servers. By the time you subtract all the expenses you won't have as much income as you started out with. For this reason, you should consider not trying to be the cheapest hosting company around as you won't really earn well enough to stay in business.
It's really difficult to give an answer to this question. It depends on many factors -on what type of hosting services you would like to provide, on whether you provide people with good and timely support...it depends on your location even, if you want to develop your business locally. I don't own a web hosting company, but i've read that people have about 10-100 clients at the beginning. Your revenues depend on the prices of your services.