Five seems like a good max at one sitting. Leave it at that for today; if you get through until tomorrow without discomfort, them you can up it by one or two if you wish.
(I ate three prunes at lunch in third grade, and threw up abut two minutes later. I found my limit.)
I’ve eaten seven on a bowl of bran flakes and did not suffer any ill effects. Consider that the average adult eats 15 to 20 grams of fiber a day and 25 to 30 are recommended. I would advise you not to eat more than seven at one sitting. Stretch them out over the hours of the day.
I would say that 5 prunes per day should be the limit. Here is an article discussing possible side effects from eating prunes.
I like to put dried fruits in a cup of water and leave them in the refrigerator. The water is absorbed, giving the fruit more bulk when you eat it without adding calories.