Agree with above posts. Corydoras are very social fish, almost if not just as much as are loaches, but in somewhat different ways. But the fish "expect" to be in large groups and it is actually unkind to not provide them with several of their own; you cannot see stress from something like this, until it has developed to the degree that the fish clearly show it, and we don't want that. Though with cories (but not loaches, just so no one gets the wrong idea) mixing species doesn't seem to bother them from what ichthyologists can ascertain. But never have fewer than five, and when the tank has the space as a 55g does, a much larger group will improve their health and "happiness." I agree with whomever suggested 12+. I had 50 in my 70g, and now have 40 in my 40g [had to downsize tanks when I moved].
This really is extremely important and crucial for the fish.
Another point concerns food. Cories cannot manage on "leftovers," and they must be fed the sinking foods to ensure their health. Shrimp pellets are particular favourites, but most good quality sinking food intended for bottom feeders will help. I use Omega one Shrimp Pellets, Omega One Veggie rounds (the added veggie matter is important for a healthy digestive system in all fish) and Bug Bites (these readily sink). Frozen bloodworms are relished, but should only be fed once a week as they can cause digestive problems. By "frozen" I mean fresh frozen and you thaw them out before feeding, as opposed to freeze dried which is best avoided.