While the original "product listing grid" mod had a user-selectable format choice, its implementation was clunky, so it was excluded.
While one can find lots of sites that offer user-selectable increments, in practice I find that the best usability is to set a reasonable number and just be consistent. Often more is better, unless your images are poorly optimized ... in which case, optimize your thumbnails and set the qty per page to be something like 50, which lets users get the big picture easily, and not be confused with too many choices on-screen.
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