It is really hard to control for all the variables (price, motivation of seller, location, who the listing agent knows, what style of home is hot), but all other things being equal, it's better to have more photos for a listing. In fact, since it is one of the easiest things to control (it's a lot easier to snap 25 photos than change the price point of a house) I'd argue it is one of the things home sellers should expect.
Would love to hear your suggestions on making this more rigorous. Off the top of my head, it'd be interesting to list a set of same homes twice, once with few photos and once with many photos, and see what the difference in response was. Unfortunately, that is a bit beyond my scope for the time being.