How Many Apples Do You Actually Need For A Pie?
As with many things related to baking, the art of baking a perfect apple pie is an incredibly inexact science. There are many common mistakes to avoid and, even if you were to follow all of the tips for the best apple pie you can find, many recipes will list wildly differing amounts of apples needed for the same size crust, leaving you wondering how many apples you actually need for one pie. The short answer? For any standard 9-inch crust, you'll probably need no more or less than 6 to 8 average-sized apples.
The long answer is that it really depends on what kinds of apples you're using for your apple pie filling. Some apples keep their shape much better after being baked, while others will simmer down to a much smaller and mushier size. Firm apples like Granny Smith and Honeycrisp apples will probably require less to fill up a pie, while softer varieties like Golden Delicious might need an extra portion to make sure the pie doesn't collapse too much after baking. And, don't forget to account for pie size: If you shift one pie size up or down, you'll want to add or subtract roughly one apple.
Other factors to consider
The general size of the apples you choose and how you cut them for the filling are two major aspects to take into consideration. Around 5 to 6 cups of thinly sliced, firm apples seems to be the sweet spot for a 9-inch pie. Generally speaking, a medium apple around 2 ½ to 3 inches in diameter will translate to about one cup when thinly sliced, minced, or otherwise finely chopped. If you're preparing them in blockier portions (like thick slices or cubes), you might end up with roughly another ⅓ cup from one apple. In simple terms, the finer you cut the apples, the more you'll need for the filling.
Of course, converting smaller or larger apples into the same measurements involves a lot of approximation. While you won't know for certain until you're actually cutting and prepping the apples, you can make a rough mathematical estimate that a large apple between 3 ½ to 4 inches in diameter will yield around ⅓ or ½ more than a medium apple would. Meanwhile, a small apple around 2 inches will yield roughly ⅓ cup less.
However, another complicated aspect of trying to measure out apples is the fact that they reduce at varying rates while baking. One way to cut out as much of the guesswork as possible is to pre-cook your apple pie filling to get most of the reduction out of the way. This will let you accurately eyeball the filling once it's in the crust.