Can I legally log hours flying RV-12 towards commercial airplane rating?
Does RV-12 qualify as 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.