Probably 50/50. I can get 9 in after work on Monday and Tuesday. Wednesday and Thursday are usually too busy to play. And I usually play 18 on Friday and Saturday. On vacation days it’s 18. And in the winter mostly 9 I think.
I very rarely play 9 holes because (1) it doesn’t seem worth the travel (and in winter getting your kit dirty) for just 9 holes and (2) for me, golf is about playing 18 holes. However, I am retired and so I don’t have the same time pressures that working guys have. I may well have played 9 holes more often if I had taken up golf while I was still working.
Very seldom play nine holes, not involved in any leagues. Most of mine are due to rainouts. Once in a while I’ll only plan to play nine, like once a year or so.
Most local courses it’s only about 5.00 to play 18 than it is nine, so that factors in too. Plus if I’ve got a solid 9 going it’ll be tough to stop at 9 if I need to be someplace else.
Most of my rounds are 9, as most of my golf these days are after work 9's or late weekend rounds. I'd say somewhere around either 60/40 or 70/30 being 9 vs 18. 2-2 1/2 hours is way easier to fit into the daily schedule than 4-5.
The only time I play 9 is in the league I'm in or when I play with the wife. So that puts me over 20 times a season. I'd much rather play 18 which I do on days off or weekends. Although sometimes I wish I had quit at the turn when the round falls apart on the back nine.