The frame I built this spring is 1x9, but only because I had all parts but the shifter already in hand. Ten-speed is more mainstream, and I would have gone ten-speed for sure had I been buying all new parts. (Because staying in the mainstream is easier, and because I see nine-speed as fading away and do not want to invest in more of those parts). Given an unlimited budget, I would have gone 11-speed just to try that out, but my budget was limited, and I had most of the nine-speed parts, so I went that way.

FWIW, fewer speeds make derailleur adjustment less finicky. Eight-speed bikes are a joy to adjust. But the market trends are toward 10- and 11-speed, and also toward 1x setups.