Two or three are enough, four if you're really starving. They're cooked perfectly and juicy every time I've had them.
What makes this place great is that not only are the burgers fresh and homemade, some of their sauces are homemade as well, including the jalapeno and smoky chipotle sauce. They also have BBQ sauce and ketchup.
Marks has some good, crispy fries as well, and also goes great with the above-mentioned sauces.
They place is basically a bar and they also serve beer, but also milkshakes....the next thing I have to try on my next stop is their Guiness Shake! This is the exterior of the restaurant...just look for the bull overhead and their sign touting that they have the best sliders in the city.
(between 3rd Ave & Astor Pl)