How many grams is 500 ml of protein? You would know by now with this diet if you were lactose intolerant or not - which, btw, has nothing to do with the protein. It sounds like this should be fine, unless the shakes are a few liters each.
I've heard up to 6 g of those is okay. You should be able to take, then, 20 pills without issues. 20 may be overkill... I usually take a couple grams per day.