The one that works, however this is a question we always get asked, if we start with shot size the minimum for game has to be 6 shot, this size carries sufficient mass/impact for birds up to 30 yards, for longer range no 5 shot is a must as it is important that lethality is a priority )larger shot greater mass/impact).
All too often persons with smaller shot will injure birds by taking a shot that is beyond the lethality of the shot being used resulting in game being hit but not killed, something that must be avoided.
As for how much of a given shot size well that is mostly down to choice but generally 28-30 grams is sufficient, greater volume such as 32/36g can give a better shot density within the "pattern". BASC provide an excellent course called "Patterning For Lethality", we will be delivering a similar training package in the new year as part of our soon to be launched training course for new game shots (first course April 2020).