If you are planning to start a normal hosting without much extra or attractions then getting 20 customers per month for a startup seems to be un realistic. I will suggest you to have a good market study and customers latest requirements for hosting, Setup the plans according to the statistics, Try to provide some thing unique, and since you are in development, I will suggest either some plguins, or some deveopmental support hours for clients who host with you etc which will attraact a lot of new customers who might be not interested to contact developers for the site setup. Also an discount offer in developmental rates for those who host with you. I know many hosting company who develop the sites. host those wither in the shared hosting or in a seperate vps etc.
Unless there is any thing unique, getting traffic and getting 20 customers will be tedious and I will say almost impossible.
Some other methods are be active in Forums , Get reviews from your existing customers and try to provide some referal bonus for the existing clients, etc can improve the sales but not as per your wish. Those will take some time.