Why are there 18 holes in a round of golf?

The early golf courses had no fixed number of holes. Some were 5, some 7, some 12. At the Royal and Ancient Golf Club of st. Andrews, we had 11 holes that went straight along the water. they'd play 11 out, turn around, and play the same 11 back. A round was 22 holes.
At Oct. 4, 1764, the first four holes were too easy, so they made them into two. That made 9 holes out , 9 back , 18 holes.