The races with the highest number of winners on the grid were Belgium and Spain 1978, with the following 15: Andretti, Brambilla, Depailler, Fittipaldi, Hunt, Ickx, Jones, Laffite, Lauda, Mass, Peterson, Regazzoni, Reutemann, Scheckter, Watson. None of Brambilla, Mass or Regazzoni had qualified for the previous race (in Monaco) which was Depailler’s first win, and Jacky Ickx did not qualify for the following race (in Sweden).
Conversely, 3 races have not seen a single winner on the grid (excluding future winners of course): Britain 1950 (the first World Championship GP), Indianapolis 1950, and Italy 1960 (several teams withdrew on safety grounds).
Since then the record low (excluding USA 2005 with 2 winners) is the early races of 1995 (when the only winners were Schumacher, Hill, and Berger) before Alesi won his first race, as well as the 2 races in 1994 for which Schumacher was banned (Hill, Berger, and Alboreto were the only winners).
Thanks to http://www.statsf1.com for this!