Most of the routers in the market today have the latest WiFi standard which is 802.11ac. This standard offers higher speeds compared to the previous standard. With the latest version, the support for Antennas on routers goes up to 8! Even though the highest number of Antennas installed so far on a router are only 6.
Many people have this misconception that higher the number of Antennas on a router, the greater the speed. This is not true. There is no correlation between speed and the number of Antennas.
Antennas improve the range of your router and it also aides in handling a larger number of devices. So basically if you have a large house and the requirement is that you need good wifi coverage a cross all rooms then one should go for a router with a larger number of Antennas.