The Limber chests for the 12 pdr Napoleon, for example, carried 12 shot, 12 case, 4 shell, 4 canister (
The 1864 Field Artillery Tactics by French, Barry, & Hunt). The effective range of canister was only 400 yards (370 m), but within that range dozens of enemy infantrymen could be mowed down. Even more devastating was "double canister", generally used only in dire circumstances at extremely close range, where two containers of balls were fired simultaneously.....Now, after firing one canister round, how much farther would/could attacking infantry advance before a second round of canister were loaded?.....After the second volley, would advancing infantry continue the charge or would they break?.....If they continued, how far would they go AND at what point would the Battery Commander decide to limber up and withdraw?? Did the artillery have a supporting infantry unit?...What would their supporting unit do, if they were not otherwise occupied??
In my opinion, there are too many variables to say what was standard protocol, except to take each charge on a case by case basis, depending on terrain, cannoneers and horses to continue firing or to withdraw, moral of charging troops, moral of cannoneers.....and the list of variables goes on....
I will say, however, don't believe everything seen in the movies!!