Am I right in thinking that you are proposing an engineered floor board to the finished space? How big is the room and what is it’s function?
It is unlikely that your existing floorboards (assuming they are into timber joists) are fully rigid and/or flush, and you will struggle to get any board product thicker than say 6mm to fully follow these without some degree of bounce (either from the nature of the existing undulating boards, or from the inherent strength of the plywood being greater than the fixing between board/joist and pulling these apart over time).
You would be better to remove the boards and replace with 18mm plywood or OSB, best removing any huge discrepancies with the joists below with shims/packers. Figure out the difference between low and high points over the whole room and get a rapid drying Ardex or Mapei levelling compound to suit (no need to double board). Smoothness depending, a latex levelling screed on top of this with the finished engineered board adhered on top.
Answer to overall room size depending, you may want an uncoupling membrane between screed and finished board to take out any differential movement between concrete and timber substrates below. This assumes the concrete is not an old fireplace hearth that can be taken out above some of the above steps.
Potentially opening a further can of worms with existing skirting boards, doors etc but above will get you a level floor.