The WAC740 can simultaneously communicate to a maximum of three MU-MIMO-capable client devices.
At least two MU-MIMO-capable clients must be connected to the WAC740 before MU-MIMO takes effect. With a single client device connected, MU-MIMO provides no meaningful performance improvement.
Future software enhancements will increase the number of simultaneous clients to 4 MU-MIMO-capable devices.
For more information about the WAC740, see What are the features of the WAC740 802.11ac Wave 2 Wireless Access Point?
For more information about MU-MIMO, see What is MU-MIMO and how can this technology be useful in my network?