The Celts were as good as a 48 win team last season (Expect W/L based on point differential).
They added, on net, probably 4-5 wins worth of talent this off-season.
On the other hand, they have no continuity whatsoever in the starting lineup, unless Smart or Brown is starting beside Horford.
I think the lack of continuity is worth 2-3 wins.
So, 48 + 4.5 - 2.5 = 50
I think you maybe could argue for an extra 2-3 wins because the bottom of the conference is really bad this year, but I feel OK about 50.
Setting aside a very specific win total projection, I feel good about expecting the Celts to finish with about the same number of wins as the Cavs, and for the Wiz or Raps to finish ahead of them.