Why does it take 2n-1 rules to make a string in Chomsky Normal Form?

[@kaivanmayelzadeh5385]

I get why it must take exactly n-1 steps to get n variables, but what property of CNF guarantees that the n variables we derive are going to be the ones we need to yield the terminals and get the string?

[@maheshj01]

Can you please explain this part at 04:15
S -> BC ->X1 X2 X3 ....Xn ?
I didn't understand how you can add more than two variables on RHS in Chomsky Normal Form.