This has been a subject of conversation on these boards so I called Universal and asked what is the rule. I was told one card key per room and you can request more if you need them at check in and the kids do not need them to enjoy Universal Express. I remember getting two when I checked into the HRH.
In my personal experience one flash of a valid card has gotten in up to five people many times over.
We stayed at PBH June 15-17th. My DH, DD(16yrs old) and myself all had room keys. We only showed one key when using the front of the line express. Attendants noted one key lets up to 4 guests on the rides.