how no. of antennas related to the data rates !!!!

check the following URL

http://en.wikipedia.org/wiki/IEEE_802.11ac


for 1-antenna 433 Mbit/s
for 4-antenna 867 Mbit/s

how this comes !

Here are my thoughts.

Right above where it says 1-antenna 433Mbits they say assuming 256 QAM. 256 QAM means you are modulating 8 bits per symbol. The more bits you can modulate in a symbol the greater your data rate can be, but at a cost of having a really good SNR.

If you increase your number of antenna's by factor of two your can increase the amount of transmitted spatial streams (data paths) thus explaining how 867 M/bits is derived.