user28706: I know that the blow red encircled component is the 16MHz Crystal Oscillator (XTAL):

But PB6 and PB7 pins are called XTAL1 and XTAL2 pins which are not accessible by the Arduino board. 
Are these pins where the 16MHz crystal oscillator is connected. But if so why are they two of them? I searched net but couldn't find explicit information about these. 
How many clocks in total this board have?

user42539: The Crystal in Red is for the Atmega16U2 that manages the USB interface with its high speed connection to the PC (12 MHz for USB2 I think).
The Resonator in Yellow is for the Atmega328P.  It is less precise than the crystal oscillator.  
PB6 and PB7 are dedicated to the external crystal or resonator by the fuse settings used for Arduino. 
You can read more about the XTAL1, XTAL2 pins and the fuses from the device datasheet 
https://www.microchip.com/wwwproducts/en/ATMEGA328P
You can connect a crystal to the pins, but you will also need 22pF caps to connect from the pins to Gnd. And remove the resonator.  Current Uno layout does not really support making those connections.

28706: <p>I know that the blow red encircled component is the 16MHz Crystal Oscillator (XTAL):</p>

<p><a href="https://i.stack.imgur.com/RQz9o.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RQz9o.png" alt="enter image description here"></a></p>

<p>But PB6 and PB7 pins are called XTAL1 and XTAL2 pins which are not accessible by the Arduino board. </p>

<p>Are these pins where the 16MHz crystal oscillator is connected. But if so why are they two of them? I searched net but couldn't find explicit information about these. </p>

<p>How many clocks in total this board have?</p>
 42539: <p>The Crystal in Red is for the Atmega16U2 that manages the USB interface with its high speed connection to the PC (12 MHz for USB2 I think).</p>

<p>The Resonator in Yellow is for the Atmega328P.  It is less precise than the crystal oscillator.  </p>

<p>PB6 and PB7 are dedicated to the external crystal or resonator by the fuse settings used for Arduino. </p>

<p>You can read more about the XTAL1, XTAL2 pins and the fuses from the device datasheet </p>

<p><a href="https://www.microchip.com/wwwproducts/en/ATMEGA328P" rel="nofollow noreferrer">https://www.microchip.com/wwwproducts/en/ATMEGA328P</a></p>

<p>You can connect a crystal to the pins, but you will also need 22pF caps to connect from the pins to Gnd. And remove the resonator.  Current Uno layout does not really support making those connections.</p>
