Buying resources from your neighbor
You can only buy the amount that they have.  Here are the relevant rules:

The resources of a city are produced by its Wonder board, its brown cards, its gray cards, and some yellow cards.  To build a structure, a player’s city must produce the resources indicated on its card.  Example: Giza produces 2 Stones, 1 Clay, 1 Ore, 1 Papyrus and 1 Textile.
Often,  a  player  will  want  to  build  a  structure  requiring  resources  he  or  she does not produce.If these resources are produced by a neighboring city – players sitting directly to the left or right of that player – the player will be able to purchase the missing resource(s) through commerce.

You are buying resources that they produce and the rules for production indicate that each symbol only produces one resource.
Note that you buying these resources does not prevent the owner or the person to the other side of the owner from using it.  The rule is a little weird, but each player can use each produced resource once:

Selling a resource to a neighboring city DOES NOT prevent a player from using it, during the same turn, for his own construction purposes.