See, I always thought that each day you got the full list of things so that at the end you had twelve pear trees (with partridges), 22 turtle doves, 30 French hens, 36 calling birds, 40 gold rings, 42 geese, 42 swans, 40 maids, 36 pipers, 30 ladies, 22 lords, and 12 drummers.
Sure, it seems a bit excessive, but if you take the song literally, that's what you end up with.