The area of a 5x7 image is slightly less than half that of a 10x8 image. All things being equal you should be able to tone approx 20 of these to the litre. However, as gold toner starts toning in the shadows and progresses to the highlights low key images tend to exhaust the toner faster. If your image is a balance if shadow, mid and highlight tones then it will depend on the effect you want. If you want to cool the image by toning mainly the shadows then you will get more prints to the litre. If you are toning to completion then, obviously the toner will exhaust quicker. I am sure that there is some absorption into the paper but am not entirely convinced that this makes a significant contribution to the exhaustion as compared to the toning action itself. As I have never done any alternative processes I can't comment on the effect of using an untreated art paper.
I hope this helps.