Mercedes Bass had an unusual predicament as she did the seating for a recent dinner party — too many gentlemen. “Usually hostesses have to find extra men,” laughed one social. “How do you seat a dinner when it’s all men?” Sources say the guests included Picasso biographer John Richardson — who brought Chris Ely, Brooke Astor’s but ler who testified against An thony Marshall — and David Rockefeller, 93, who brought along a male friend and a bodyguard. They must have had plenty to talk about. It was Rockefeller and Astor’s other close friend, Annette de la Renta, who triggered the case against Marshall when they learned she was being mistreated by her son.