Number of trade goods in markets of all center kinds (village / castle / town) are related to prosperity values of these centers.
Village : number of trade goods changes between 1-7, average 4
Castle : number of trade goods changes between 5-15, average 10
Town : number of trade goods changes between 10-50, average 30
If you think you faced with lots of towns with 10 trade goods you can upload/send your savegame for detailed examination.