When baked in muffin tins, filling for a regular 9 inch pie makes about 16 - 18 mini pies. You will use about twice as much dough as for a regular pie though, so fill your fridge with dough balls before you start. I was lazy and only made open-topped pies. I cut down a coffee filter to make a template until I got a handle for how much dough I needed for each pie. For one mini pie, I used a ball of dough about equal to a walnut in shell. I ended up rolling each crust out individually and then pressing them into the tins. Crusts this small are very easy to roll out, and if the edges are irregular, it still looks fine. Oh, and in retrospect, Derby pie was not the best choice for mini pies; you get a much higher crust to filling ratio with little pies, so the effect here is very much like a chocolate chip cookie wrapped in pie dough. But maybe you're into that kind of thing...
The great thing about making mini pies is that you can be really flexible. I made up the first six pies just to see if they would work, and then did a whole pan using the rest of the filling. And unlike making one whole pie, you can sample these before serving. I'm also eyeing my freezer full of dribs and drabs of summer fruits and thinking about making a whole variety of pies at once. I've seen my future and it is full of tiny, tiny pies.