Apples are very good for you and you can eat as many apples as you desire.
The worse thing that will happen if you eat too many apples is you get diarrhea or an upset stomach and you'd probably have to eat around 10 or 20 apples in a day to get an upset stomach or the diarrhea from it.
But you would start feeling an upset stomach coming on if you did eat too many apples and then you'll know the limit on the number of apples you yourself can eat in a day.
Apples are very nutritious and contain fiber as well as vitamins that can help keep your body healthy and can possibly prevent some diseases.
Like the saying an apple a day will keep the doctor away.
Well not fully true but apples are very healthy for you and you won't be harmed by eating too many apples.
Sometimes I'll eat 3 or 4 apples per day for a week but after a week of eating apples I get tired of eating them.