Well, they used 11 last season, which is probably pretty typical? They probably start Luce, Prokop, and Matier in Atlanta, I would guess.
Atlanta has 3 players signed themselves:
F - Mitch Walinski
F - Cody Sylvester
G - Tyler Harmon
... but they'll need a lot more. I don't know how the protected lists and qualifying offers really work in the ECHL, but they did give qualifying offers to several other players, which apparently have a July 22 deadline.
Assuming we send Grigals, and a couple forwards (maybe Felhaber and Kondelik?) there... they definitely need a bunch more players.