I checked here too before getting the Apple Watch. Anthony's thread proved helpful with some questions, but some of the questions I simply couldn't resolve before trying it. And even after trying it for several weeks, today was the first day I could get the watchface to display the number (maybe because I'm sick, and it's the first time I've had to troubleshoot).
It's my BG being displayed by the dex app. In watch face options from your phone, search for ones that list Dexcom as a complication (there aren't many....maybe 5 or so). I scrolled through til I found one that would also display things I needed alongside it (like battery life and stopwatch).
Admittedly, I will forever miss my normal watch and occasionally switch it out for a day or so.