Damn if george the City & Aliens forum troll can't remember I really must go look it up
OK - from a post by ARK-Eponyx - not recent tho so who knows for sure? Superior markets have rougthly 120+ shops.
One shop can only be rented per account, the shop is tied in to the account not the charter you can actualy restrict it to be only accesible for all your charters on that one account only, guild only or free for all.
Each shop can max have 30 items in it from that account that owns it.
SO I was wrong about 21 items - but 30 isn't massively more