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I'm contemplating a static channel for a high density space and am curious about avoiding adjacent channel interference in 5 GHz.  Is ACI an issue with 20 MHz channels?  If so, at what received amplitude should I be concerned with ACI, so that I can create channel plans with it in mind?

The 5 GHz channel plan was designed with proper channel spacing for 20 MHz wide channels in the first place.  That is why the channel designations are in increments of 4, e.g. 132. 136, 140.   Each of these is 20 MHz apart, unlike the 5 MHz spaced channels in 2.4 GHz.

Theoretically,  you should have no problems with adjacent channels, but be aware that any two adjacent channels close enough physically and set at high enough power levels still DO overlap at least a little bit.  The out-of-channel portion of the signals do not just magically stop at the edges of the 20 or 40 MHz Spectrum mask as defined in the 802.11 standard.

In any installation, even a small one its always best to place the AP's channels as far from each other as is possible, but of course in a high density installation the design takes more care.   There are websites discussing the best practices for AP layout in the 5GHz bands.   Generally speaking they suggest at least two channels between all  neighboring AP's, and two cells between AP's on the same channel.

If you are going to need location tracking, you'll want the perimiter covered especially well.  Without that, the system accuracy will be poor.  This idea is sometimes expressed as covering Convex Hulls. 

I would start with an accurate floor plan, and use a predictive planning tool to make initial estimates for AP placement.   Then follow that up with a site survey to finalize the design.

With multi-floor installation, you do not want to stack AP's on top of each other, even if on different channels.

Sorry I couldn't give you more specific guidelines.

PS.  I have heard  nothing but horror stories from people who use dynamic channel selection.   I'd say static configuration  is the way to go.