I asked a friend who does protection for a different US agency and has worked along side the Secret Service to play this game. His number is 5: the 2 dudes directly behind and the 3 a few rows back. He said the chick in the Dayton shirt has the "look" but does not think she is an undercover SS agent. He said for an event like this and the size of the crowd shown and proximity to the POTUS that that is a normal number, there are however a lot of people not in that picture both in normal SS attire and undercover. He said there are also British agents there to protect their asset but does not see any of their usual "look" in the picture.
PS His best guess as to the people on either side of the dignitaries are that they part of what ever delegation set up the event and/or part of the British travel group.