While it depends on the size of the apples, recipes I’ve seen call for 3-6 fresh apples cut or diced to make 3-7 cups. The average 9″ pie plate can hold about 4-5 cups of heaped apples. Apples vary greatly in size. Some varieties can be double the weight of other varieties. The best type for a baked Apple Pie in Australia (where I live ) is considered to be the’Granny Smith’ They are an average size type of apple and I have made dozens and dozens of pies over my lifetime and I ALWAYS USE 5 peeled and cored Granny Smiths or ‘Fuji’ variety. Granny Smiths do not turn into a ‘mush’ when you cook them.
Never cook them for more than 6 to 7 minutes (gently simmering)

How to Make Easy Apple Pie by Allrecipes