Q. Is there a maximum number of recipes you can add?
A. There is no reachable limit on the number of recipes; you can add as many as you want although it is limited by computer power.
Q. Is it possible to create recipes that take an item with a stack size of more than one?
A. Unless you create a custom crafting table, this is impossible.
Q. Are there other files like this in Forge?
A. There are many files in Forge that change simple small things. Look around the code a bit if you are looking for something specific. These were just commonly used once.