I don't think there's a "limit" but they are around 250kcals a go so I wouldn't go too overboard on them if you want to see a loss at the end of the week. I have one about 3-4 times a week for lunch and have good losses but wouldn't have more than one a day personally.
I would agree with TheMissus - though a free food - don't go overboard on them. As a small lunchtime snack - fine or as an alternative to giving into temptation for something else.
The way my consultant would put it - though a free food - things like mugshots, muller lights, etc are artificially free - i.e. they've been prepared for us and the boffins at Slimming World have declared that they're free - against things that are naturally free - fruit, veg, meat and fish. With (most) fruit and veg falling into the superfree catagory. and to pick natural over artificial whenever possible.