[neatoman]

So I'm pretty sure we're all a little disappointed that an artbook is labeled as volume 7, I know I am. However, that does not mean it could be the last in the series. While UC was originally only concerned with the Eastman/Laird issues, volume 6 went with a lot of material they had nothing to do with. In fact, a lot of it was already published in the Color Classics

The issues mentioned in the title were part of the official canon and make up about 200 pages. The original Tales series is about 220 pages long. Then there's of course volume 2 that was published by IDW as Classics 8-10, at 350 pages. I'm not even convinced that IDW cared about the canon, given what volume 6 of UC looks like. Not counting what was in volume 6, only going by what made it into the Classics trades and isn't part of the canon, that's 530 pages of guest issues.
This makes up about 1300 pages that could be part of the Ultimate Collection.

On the other hand, Volume 5 of UC has only has 180, annotations included. Signifigantly shorter than all of those. The longest in the UC is volume 1 at 319. If you divide those previously mentioned 1300 pages by 320, you's get 4 volumes.

My point is, you could have between 4 and 7 more volumes, without resorting to making an artbook. Possibly even more.