The quantity word "座" may refer to the incense burner (一座香炉)where joss sticks are held vertically while being burnt, or a spiral coil like a pyramid or mountain. The latter allows more time for the joss stick to be burnt. As known a custom in temples, time used to be measured against the duration of the joss stick burnt out.
Since there isn't a strict idea about how long the duration of a burning joss stick is, I guess there is no need to specify the detail of the quantity word.
For the coil incense:
http://www.huitu.com/photo/show/20111226/191116237148.html