And again, like many developments in golf, the standardization of 18 holes can be credited to St. Andrews.
Prior to the mid-1760s - and right up until the early 1900s - it was common to find golf courses that were comprised of 12 holes, or 19, or 23, or 15, or any other number.
Then, around 1764, the linkes at St. Andrews, Scotland - what we now know as The Old Course at St. Andrews - converted from 22 holes to 18 holes. The reason? Well, everyone knows 18 holes are easier to take care of than 22!
Eighteen holes did not become the standard until the early 1900s, but from 1764 onward, more courses copied the St. Andrews model. Then, in 1858, the Royal & Ancient Golf Club of St. Andrews issued new rules.
I'll let Sam Groves, curator of the British Golf Museum who helped me with this explanation, take it from here:
"In 1858, the R&A issued new rules for its members; Rule 1 stated 'one round of the Links or 18 holes is reckoned a match unless otherwise stipulated'. We can only presume that, as many clubs looked to the R&A for advice, this was slowly adopted throughout Britain. By the 1870s, therefore, more courses had 18 holes and a round of golf was being accepted as consisting of 18 holes."