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Thank you for answering my question. I can see how large art prints would benefit from more megapixels, I just wonder how many is enough and when does it become too many? Not to encourage more arguing here, though.....
Different folks have different opinions about when more megapixels will stop bringing resolution benefits. In theory there are some people who claim from various calculations that we may be able to go as high as 120 megapixles and still see resolution improvements. Again that's a theoretical limit, and a real world limit could be quite lower.
To look at it another way (theoretically again), an 18mp crop sensor (such as the one currently used in the 7D) if retaining the same pixel density on a full frame sensor would be something like 46mp (if I recall the result of the calculations correctly).
But in pratical terms, as more megapixels on the sensor start to impose more compromises on the camrea, how many is 'too many' will depend on the individual photorapher and what their needs are. For some 12mp is plenty. For some 21mp is not enough. That's the beauty of having different camera models. If Canon comes out with a 36mp camera and you think that's too many megapixels for you, no one if going to force you to buy it. It's not like they're going to stop making other cameras and you can always buy a different model with fewer megapixels.
Of course, when 10 megapixel cameras came out, I remember people on camera forums griping about how that was too many and 6 megapixels was enough for them. When the next generation of higher megapixel cameras came out people were saying that 10mp was enough for them and 12 or 15 or whatever was too much. Heck, people in the alt forums right now are wringing their hands over the NEX-7 having too many pixels and wishing that it had fewer pixels like the NEX-5N because of "more noise" when in fact it's been shown in side by side examples that the difference in noise is completely trivial up to ISO800 (and if you downsample the 7 files to the size of the 5n files most of the difference at very high ISOs disappears as well).
Point is, we may be approaching a point of diminishing returns on higher megapixels, but that doesn't mean we've hit the wall just yet.