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.

Also explored in an awesome video by Vihart: YouTube/Piped/Vimeo