Par for 18 holes varies by course. However, most courses are between par 70 and 74.
Each hole on the golf course is given a score that is considered "par." These are usually par 3s, 4s, and 5s with some rare par 6s too. To reach the overall par score of the golf course the 9 or 18 holes are added up to make par. Each side of the course whether the front 9 of back 9 is scored separately in order for 9-hole rounds to be played. However, our main focus on 18 hole rounds leads us to looking at the overall par score. The vast majority of courses have one of the 3 par scores: 70, 71, or 72 with a few having 73 as well. These scoring totals come about based on the basic layouts of most courses around the globe.
The average layout of a typical golf course includes mostly par 4s, a few par 3s, and a few par 5s. The prototypical par 72 18-hole golf course has 4 par 3s, 4 par 5s, and 10 par 4s adding up to create the most common 72. However, many new courses being built or based on the space the course is located on are different as we can see on this par 71 scorecard shown below. It is a shorter course that may have a space constraint as there is an extra par 3 on the back nine instead of a par 4.
Courses with par 70 or 71 that include less par 5s are becoming more common as well as some courses adding par 3s instead of par 4s lowering the total par score. At the end of the day it is all based on the individual course that the golfer is playing at and the layout of that specific course. Whatever number each hole's par score adds up to across the 18 creates the total par score.