I think that one of the reasons why there has been so many views and not so many answers is that this is not an easy question to answer.
So, without changing anything, I think that taking on one person every 2 week period would be pretty reasonable (adding a week or so to the due date of primips might be more accurate when planning). But I think that trying to change your situation might be a better option. Here are some ideas:
Find a backup, even if she is several hours away. Offer to pay her an unreasonable amount of money to come to your labors so that she will truly be willing to get up in the middle of the night and drive to where you are. It might break the bank if you have to do it very often, but you probably won't. Building a relationship with her might also help strengthen your position in your own community. I have found that once a midwife has worked with me in a backup capacity she is more likely to refer clients to me, even if we have only talked on the phone.
Train your apprentice in labor-sitting. Teach her how to do vaginal exam, take vitals on the mom, heart tones on the baby, assess for normal bleeding, and how to judge contractions. If you do have two labors at the same time, you can send her to the slower/earlier one. If you absolutely have two women birthing at exactly the same time, you might be faced with transporting one. In the short term, until you find another solution, you might want to present this possibility to your clients so that it is not a huge surprise when it happens.
Consider asking your clients to labor at your home in the rare case of two women in labor at the same time. Yes, I know this is probably illegal, but I think it is defensible as an emergency plan.
I think one of the important things is to make a plan for what you would do if there are two women in labor and share that plan with your clients early in their pregnancies so that they have a sense of what to expect. If the choice is between using you and planning a hospital birth, they might still choose to use you and take their chances. I think that I would.
You are right, though, about it being unlikely that two women would be in labor at the same time, even with a client load of 4-6 per month or higher. I think that if you have some plan in place, you can safely take many more clients than you do now.
Good luck in your growing practice!
Stacia -- intrepid mama, midwife, and doula. Changing the world one 'zine at a time.