I suspect the highest numbers would probably include Easley, Elmore, Caldwell, and Wayne Reynolds. The artists that would qualify for this would need to be people who worked across multiple editions, and there are some artists that are mostly known for a campaign setting or part of an edition.
Easley and Elmore did art for the BECMI sets, and interior work on 80s modules. Easley was also the big cover artist from about 1983 until well into 2e, and even had a few covers during 3.0. Easley, Elmore, and Caldwell did a good number of color pieces that often featured as full page art in the various 2e rulebooks (though by the mid-90s in particular there was a good deal of recycling). Elmore and Caldwell had a good number of Dragon covers over the years too, Caldwell I think got started earlier, but Elmore had a longer run on the magazine, all the way to the final issue. They did a good number of novel covers too, which muddles the count a bit, because even if you don't want count the novels, that's some of the art that got recycled, and sometimes quite a bit. And all three of them worked on Dragonlance. Parkinson I think was the best of the "big four" artists but his work wasn't as prolific as the other three.
I wasn't thinking of Holloway but on reflection he had a pretty long stretch of work in the game, and probably counts too.
Wayne Reynolds is the big artist from 3e, 4e, and Pathfinder, and even had a bit of work in late 2e, so he's definitely in the running. A lot of his black and white art in the 3.0 splats got recycled though, sometimes even worse than TSR's recycling in their decay.
You guys left out Arnie Swekel. He did a lot of black and white art in the 2e days, and worked well into 3e. He did the illos in the Encyclopedia Magica, and keep in mind that work totals well over 1600 pages. He had more art in the 2e spell compendiums, though some of that was recycled from the EM. A lot of black and white art in modules and various accessories, probably most notably Night Below. He did a good number of the first chapter pages in various 3e hardbacks. Some of his work was color too, but it was less common. I think his work gets overlooked quite a bit, because he doesn't get mentioned a lot in D&D art discussions.