What we list in our datasheets is the max capability of the hardware, ignoring regulatory restrictions (there should be a note for that caveat).
Regulatory rules will mean that we may need to enforce additional restrictions, which will vary by country. For example, most countries simply will limit the total amount of power (expressed in an aggregate EIRP level) that can be transmitted in a given band. There may be other regulatory limits (band-edge or out-of-band emissions for example) that force us to further limit the power.
This is all taken into account by our software.
So in your example, if the regulatory limit is 20dBm, you would typically have to subtract both antenna gain and MIMO gain to find the max setting for the per-chain conducted power limit. Which in this case will be far less than what the hardware is capable of.