As many as they want and can afford. I don't really care about regular potions of healing, any decent sized city is going to have them and they can always craft them. Greater healing potions are available in larger cities.
I really don't worry about things like this, I rarely provide magical treasure, I just give out treasure instead and let them buy from a curated list of magic items. Potions of healing, even if using them only take a bonus action, will never keep up with the damage being dealt in my games.