Depends on how the song is interpreted. The intention is probably “by the 𝑛ᵗʰ day of Christmas, my true love had given to me [list of 𝑛+(𝑛–1)+…1 items]” but the actual grammar means that by day 12, you’d have received 𝑛(13–𝑛) of the 𝑛ᵗʰ item, or
12 drummers drumming
22 pipers piping
30 lords-a leaping
36 ladies dancing
40 maids a-milking
42 swans a-swimming
42 geese a-laying
40 gold rings
36 calling birds
30 French hens
22 turtle doves
12 partriges in pear trees
Total is 184 birds. By day 7, only 69 birds, up 50 % from 46 by day 6. At least the number of received birds stays constant (23) on days 8-12. The geese technically-a-reproducing are not accounted for, as the eggs might not be fertilized and take several weeks to hatch.