I'm booking a week long trip at a beach resort in the US and the largest room says it sleeps 5 people. There will be 6 of us. But one person will just bring an air mattress because it is a VERY spacious suite having 2 separate rooms. I am wondering, will the staff at the hotel confront us once they notice that 6 people are coming in and out often? or will they not even care?
Unless you mean something different by 'resort', you are usually paying for more than just sleeping accommodation - food, use of beach, sports etc. Generally in such places you get a wristband that entitles you to access facilities and they will definitely notice and refuse to give out six wristbands.
If there are no facilities other than the room they may or may not notice but will probably still object if they do.