24x is more than 1 ply. Typical branching factor isn't so far from 24, and with alpha-beta the effective branching factor is more like sqrt(nominal branching factor), and I think modern engines are pretty good at pruning and extending the tree to make the scaling even better than that.
A ply is half a move. Or to put it another way, one move is one ply by white and one ply by black. This is a term that is common to all sequential games.
