One thing is the Radio, the hardware involved in transferring data in some frequency, usually Router with 1 Radio have 2.4ghz, with 2 Radios have 5ghz (also called dual-band) and with 3 Radios (rare) also have (I think) 20mhz...
Another thing is the quantity of antennas, There are Routers with internal antenna, 1, PCB, or 2 (metal clip), 3 or more...
Also with external fixed Antenna (with internal cable soldered) and with rp-sma or simmilar connector (with internal cable soldered)...
Why more than one antenna?... Because of signal polarization... Think about Wireless signals as a plane sinusoidal wave, or even, like a wire/string in sinusoidal form... that wave can be Horizontal or Vertical... (there is also a 3rd habitual form as helicoidal too, but is less common than the Horizontal/Vertical)
That wave bounces around, and have different degradation depending on different materials, on air is minimal, on wood is more, in glass is even more and in walls is much more, on metals like cooper, steel or aluminum is a lot. (that because signal is very decreased from one room to another, specially on 5ghz... but not so much on 2.4ghz)...
To increase physical space coverage of propagating waves the Wireles Routers have more antennas, with different polarization, 1 Vertical, 1 Horizontal, others may have 2 Horizontal 1 Vertical, and other have more combinations, others tend to test turning one antenna on some angle, instead of being the both vertical positioned, put one horizontal and one vertical, or at 45º too... in some cases this helps, but not because physical position, because the polarization is done at electronic level and not at physical antenna positioning level... Moving physical antenna you just help to signal to bounce on other direction.
About polarization: https://www.google.com/search?q=signal+polarization&source=lnms&tbm=isch
About antenna Angle: https://www.google.com/search?q=router+antenna+angle&source=lnms&tbm=isch
MIMO is a different thing, it means that the radio is capable to have multiple full streams of transmission at the same time, usually this is very useful for many devices and MIMO capable devices.
It is true that mutiple path in wireless signals are related to have multiples antennas but is not this case... think for example in a modern cellphone or notebook, with internal antenna, probably have only one for that frecuency, but is has a MIMO capable radio chipset that could open multiple data transmissions path on same frecuency and use it to increase practical bandwitdh...
I am not an expert about this, but AFAIK my explanation is very accurate, hope it helps...