I intentionally don't count exactly how many scenes they affect, but I can assure you, it's plenty for both testosterone and estrogen.
Yeah, character romances can be locked if you have too much of one of the two (for example a romance that would have testosterone checks might automatically be disabled if you have high estrogen, and vice versa). Don't take that as meaning it's better to keep both low though, as you'll be missing the higher rewards of the romances (usually translated in perma-stat gains for both participants). Most h-scenes in the game actually have extensions with the juicier stuff and even more stat bonuses if you respectively have enough estrogen/testosterone.
Just pick what you'd normally want for your character, lorewise.