The number of teeth is mostly irrelevant with dado blades as each side blade cuts the shoulders as cleanly as they can and the chippers chip out the material in between.
Plywood and Teak are possibly the worst choices of material for tooling longevity. If you plan on cutting a lot of it it may be worth investing in an adjustable groover from Whitehill instead, a lot more expensive and typically less capacity than a dado blade, but the carbide inserts are made from a much harder grade than tipped tools and are easier to replace than constantly having to have a dado set re-sharpened.
