Information about the current offer is provided to the left of the offer sheet.  There are two reasons to accept an offer. Either the offer serves a diplomatic purpose--meaning that acceptance improves the relationship of the offering and purchasing nations; or the offer fulfills a need of your warehouse. The most useful offers achieve both these goals.

In order to gain the diplomatic benefits of trading, you must open a trade consulate in the other nation during your turn (before trade offers are made).  If you decide to accept a deal only for diplomatic gain, there is no reason to purchase more than 1 unit of the commodity. The fact that a deal is made supplies the benefit, the quantity purchased is irrelevant.

In cases where your warehouse actually needs the commodity you should be guided by your actual needs and the number of cargo holds you have remaining for trade this turn.