Can I log time in an RV-12 towards a commercial certificate, and is it a TAA?
Assuming that you are asking about the US, there are two parts to your question-total time and Complex/Turbine/TAA time.
As to whether the RV meets the requirements for a TAA that depends on what instrumentation/avionics it has. If it meets the definition, then you can use if for the training part of the aeronautical experience.

§61.129   Aeronautical experience.
(ii) 10 hours of training in a complex airplane, a turbine-powered
airplane, or a technically advanced airplane (TAA) that meets the
requirements of paragraph (j) of this section, or any combination
thereof. The airplane must be appropriate to land or sea for the
rating sought;

Further down in the reg it tells you what a TAA airplane is:

(j) Technically advanced airplane. Unless otherwise authorized by the
Administrator, a technically advanced airplane must be equipped with
an electronically advanced avionics system that includes the following
installed components:
(1) An electronic Primary Flight Display (PFD) that includes, at a
minimum, an airspeed indicator, turn coordinator, attitude indicator,
heading indicator, altimeter, and vertical speed indicator;
(2) An electronic Multifunction Display (MFD) that includes, at a
minimum, a moving map using Global Positioning System (GPS) navigation
with the aircraft position displayed;
(3) A two axis autopilot integrated with the navigation and heading
guidance system; and
(4) The display elements described in paragraphs (j)(1) and (2) of
this section must be continuously visible.

As for total time, read the regulation. An RV is an airplane so as long as you have at least 50 hours of the 250 in an airplane then you are good. You could do all 250 in the RV if you wanted.

(a) For an airplane single-engine rating. Except as provided in
paragraph (i) of this section, a person who applies for a commercial
pilot certificate with an airplane category and single-engine class
rating must log at least 250 hours of flight time as a pilot that
consists of at least:
(1) 100 hours in powered aircraft, of which 50 hours must be in
airplanes.…

You are no longer required to take the practical test in a complex or TAA airplane, so no matter what instrumentation/avionics it has you can use it for your practical.