I tell the truth! I would tell them I have an office in my home, and I don't have any clients as of yet so they will be my only provider as of now.. which means they get all your attention (don't tell them the last part..lol).
I also let them know that even though I just started my own company I have been doing billing for xxx number of years which is why I started my own billing company.
Don't focus so much on how many clients you have or don't have because it could be a negative if you have to many clients in their eyes.
Focus on you, your skills, experience and expertise. Sell yourself after YOU will be doing there billing..
Hope this helps!