There's a formula for this:
- a is the maximum number of Berries that can appear on the plant
- b is the minimum number of Berries that can appear on the plant
- c is a number randomly chosen between a and b, inclusive
- d is the number of growth stages during which the plant was watered
I think the maximum is six, throughout my whole Pokemon Platinum experience I've only been able to grow berry plants with two berry plants on them.