Today I learned trade volume in the market info screen is measured in credits.
Building on this I created a crude equation that equates trade volume to cargo units and then to Atlases.
Equation:
Spoiler
Trade volume divided by average price per cargo unit. Divide by 2000 and you get the number of Atlas superfreighters you need to carry the trade volume. The average price is the average of all buy and sell prices in the commodities section of that market.
T/(P/C)/2000=A
I then sampled several markets:
Spoiler
Market: # of Atlas
Jangala: 15
Asharu: 0.2
Sindria: 30
Cruor: 0.4
Volturn: 37
Tartessus: 17
Baetis: 4
I never realized Volturn and Sindria are such huge markets, although that does explain why the Sindrian Diktat has such huge fleets.
Also, my samples were taken in SS+, but a glance through the same markets in vanilla indicates the values should be similar.