I feel like I'm repeating myself with this answer:
In theory if you reduce supply (fewer ad slots) price will go up. But! at the same time you will reduce your revenue generated by the ad unit that you removed, and in my experience the marginal gain in impression RPM achieved by reducing supply was not enough to compensate for the revenue lost from the removed ad unit. When I did this I went from 3 ads to 2.
You can now turn this whole premise on its head, if by eliminating one ad it reduces revenue, then adding more ads per page should increase revenue. This may in fact be true, but with diminishing returns. In other words, one more ad may increase page RPM by $1, adding 2 by $0.50 and adding 3 maybe nothing (values are arbitrary, simply to illustrate diminishing returns*). So if you have 4 or 5 ads per page, then removing one could increase revenue.
The other aspect to consider is that too many ads on a page may cause Google to reduce your ranking in the SERPs, thus reducing traffic, so you will earn more per page impression but have fewer page impressions and of course the opposite would then be true too. The issue with this is attribution, traffic can change for a wide variety of reasons so can one really be sure that adding/removing ads resulted in a drop/rise of traffic.
And there is also the impact on active view viewable, but that was the point of my last post on this topic here:
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webmasterworld.com...]
*Diminishing returns explained in full economic nerdiness:
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en.wikipedia.org...]
