I have recently submitted my application to USNA, and I asked my BGO about chances of appointment and whatnot. He told me that each congressional representative and senator must have five nominated students at each academy at a given time, and ten nominations are given for each open seat available. I have received a nomination from my representative and one of my senators; both of whom gave out ten nominations. Does this mean that only one of us from each pool of nominated candidates can attend? Can more than five students attend USNA from the same district at a time?
Each congressional district can have 5 midshipman at the academy At any given time. Plus another 10 from the 2 senators. Plus others from other sources such as the VP the President and the JROTC program.
Of course each year each congressman usually fills just one spot
There are many cases of twins going to the academies at the same time. A few years ago there were twins and their one year older brother who went (older brother did the prep school for a year) to West Point.
There is a wealth of information on how the nomination process works on the SA forum: