So far I haven't been able to find more than 10 glances that I find useful enough to keep around
I'm curious as well about the load it might have on the watch, although with the 10 I currently have in there I still end up with plenty of battery at the end of the day. After seven weeks I still have yet to decide when and how glances are useful to me. At first I was frustrated by having too many, because it would take time to scroll to the glance I wanted to see, and then scroll back to a different one. When you have to scroll a lot it starts to seem faster to just go to the app itself.
Now I think I browse glances kind of like I browse headlines rather than treat them as a quick way to access specific information (I use the complications for that). I keep the glances to items that may change throughout the day, and I browse them when I'm waiting for something for someone. If I see something that makes me want to know more, I tap to go into the app or look at it on my iPhone. I do try to keep it at the beginning of the list so I can access my two most frequently used glances: Quick Settings and Now Playing plus a few others I like quick access to. The rest are just there to browse. I don't think I have ever had more than 15. Didn't notice any impact on watch or battery performance, but I nixed a few that weren't useful.
Sean