Looks like there is a painted turtle (Chrysemys picta)(left side with red on the marginal scutes) and a Yellow belly slider (T.scripta scripta) (leftmost turtle with wide stripe curving up to the eye) in there too. Actually I think the turtle in the low centre that is still halfway in the water is an Eastern Painted (C.picta picta)(smooth scutes in single rows). Probably the other C.picta is an Eastern as well but I can't see if the scutes line up.
What is the collective term for turtles? A pack of turtles? A herd of turtles? Does a herd of turtles have a very slow stampede?
A group of turtles can be called a bale of turtles although I suspect that this might be specific to (tragic) contexts where they are sold for food.
Where was the picture taken? New York City residents, b.t.w., can see ... multiplicities... of turtles like this at the rocky edges of the pond by the "Castle" and Shakespeare Theatre in Central Park.
Agreeing with MattK, there's at least one yellow-bellied slider (Trachemys scripta scripta), and the low center turtle is definitely an eastern painted turtle (Chrysemys picta picta). Most of the turtles are red-eared sliders (Trachemys scripta elegans). The turtle furthest to the right appears to be a red-bellied turtle (Pseudemys rubriventris).
This was taken at the National Zoo. Thank you, all, for helping with the identification. I don't really know turtles well and yesterday was so busy I did not have time to look things up.