Understanding RB in 5G

Let's Understand  SCS first.
As per the below table the subcarrier spacing is in the multiple of 15kHz .

 

Below is the NRB available as per the table provided by 3GPP Specification 38.101-2

But lets understand how to get 66RB/NRB/PRB from 100MHz with 120kHz subcarrier spacing  

To understand this we need to understand the concept of guard band.
Guard band is the spacing between two 5G band, this is to avoid interference between the band.
Below table provided by 3GPP Specification 38.101-2

The formula for calculating the guard band is as below.
Calculation for (CHBW x 1000 (kHz) - RB value x SCS x 12) / 2 - SCS/2
Now we will calculate how 66RB/NRB/PRB from 100MHz with 120kHz subcarrier spacing
As per the above calculation 
 (((100*1000) - (66*120*12))/2) - (120/2) =  (100000 - 95040)/2 -120/2 = 4960/2 - 120/2 = 2480 - 60 = 2420

2420 value is in kHz and need at the beginning and at the end of the band to guard it. 
Below table provided by 3GPP Specification 38.101-2

Now for 100 MHz in kHz is 100,000kHz

If we remove the guard band from the beginning of the band and end of the band then we get 95160kHz

100,000 kHz - 2420 kHz - 2420 kHz = 95160 kHz

Each sub carrier is of 120kHz which leads us to 793 sub carriers out of 95160 kHz 

95160 kHz /120 kHz  = 793 sub carriers

One RB consist 12 sub carrier.

so from 793 we get 793/12 = 66.083 ~ 66RB