The amount of hard material should depend on the type of site you are selling and on the traffic you are aiming your gallery page at. If its not appropriate to the genre/niche then you may discourage potential converts. Also too many or two few images can discourage sales. You want the surfer too be strongly interested but not overwhelmed or bored.
At my site I place softcore galleries and hardcore gallery links on seperate pages which hopefully helps the surfer to direct his interest (and credit card) straight to the object of his interst.
------------------
www.jadecandle.com