(Massive Multiple Input/Multiple Output) Massive MIMO generally refers to 4x4 and 8x8 antennas in an access point. However, there are multiuser-MIMO projects that envision the use of hundreds of antennas. See MIMO and MU-MIMO.
The Rice University Argos Project
This prototype is a Massive MIMO base station that uses 64 antennas. Each (white) antenna sends a narrow beam to a single user. (Image courtesy of Rice University, http://argos.rice.edu)