I wouldn't be too concerned about your cory having just one eye. They forage for food largely by smell, so she should be fine. We don't always see our corys feeding as they will also forage at night. If you see your fish lose condition, (getting thinner) that will be more of a worry. Just watch for that. (Hopefully there is no other problem, such as an infection in the eye. I would expect not, as it sounds as if it is an old loss.)
Also, five or more would make a nice group, but this is not essential. Two will be just fine, unless of course you would prefer more. My guess is that there are far more ones and twos of corys in tanks around the world than there are bigger groups. It would be nice to meet all these preferences of fish, but it is just not possible. Better to meet those preferences (or requirements) where they are really necessary, with more difficult fish. This is one of the (many) things that is so good about corys - they are so easy going in so many ways. Beautiful aquarium fish!