Let's say that in a 9 players game in a turn everybody preserves 8 cards.
That's 72 COMMODITIES out of decks.
During next turn let's say cities are so distributed (Early Iron Age):
4, 5, 6, 6, 6, 7, 7, 8, 9
that, 58 more cards (including calamities) out of decks.
Total 130 cards
We have 171 total commodities and 24 calamities so 195 total trade cards.
Considering that you tend to preserve your best cards, discarding or using as a filler low-value cards you can rightly think that most (say 2/3) of the 72 cards are taken from decks 5 to 9.
This means 48 cards from decks 5-9, plus 22 = 70 cards over 78.
Raffaele
Goblin's lair