A: In the past, most enterprise access points (APs) had three antennas. One antenna was used for transmission, and two were used for signal reception. Two receive antennas improve the AP's ability to recover the signal that the client transmits.
In recent years, IEEE 802.11n introduced multi-input multi-output (MIMO) antenna technology. MIMO increases the data rate by allowing multiple antennas to transmit on the same frequency at the same time. A 3x3 MIMO AP can transmit on three antennas at the same time, tripling your data rate.
Most enterprise APs use internal antennas for aesthetic reasons. However, dual-mode 802.11n APs that have external antennas often have three antennas for transmission in the 2.4GHz band and three antennas for transmission in the 5GHz band. Therefore, many APs have a total of six antennas.
You can buy APs with dual-band antenna ports. These APs allow you to use a dual-band antenna, which can simultaneously transmit in the 2.4GHz and 5GHz bands, halving the number of antennas.
The newer standard IEEE 802.11ac defines 8x8 MIMO, which means eight antennas transmitting and receiving. So you can expect to see more complex antennas in the near future.