Like the previous replies looks like a LOT of US play as many rounds possible withe the same ball.  Probably the truest statement regarding or choice of equipment:  Titleist Pro V1 or 1X.  My guesstimate ( probably will track in 2016 - in addition to the other stats:  GIR, Fairways, Putts etc. et al) a sleeve of the X's lasted 16 18 hole rounds + 16 9 hole league rounds., or 24 rounds - average 8 rounds a ball.

After almost 15 years away from the game, I re-committed myself this year.  I prefer to walk, and I prefer to play Titleist, but in the past, I was always guilty of finding several other balls (Never a fresh Pro V1, though)  along the way.  I would put them in the bag, but invariably would pull them out when I needed another ball.

So this year, my New Year's Resolution was to get back into the game and work hard to get to a 1 HDCP.  My mantra this year has been, "If i'm going to play like a pro, I should practice and play like a pro."  One of the things I forced myself to do was to only play a Titleist Pro V1 on the course.  I usually play one round, and if it makes it the entire round, I put it in the shag bag.  The shag bag contains only Titleist Pro V1's for practice to develop consistent feel around the greens and bunkers.  For putting practice, I always use new Pro V1's out of the sleeve.  Of course, since putting doesn't put much trauma on the ball, these go back into the sleeve to be used on the next round.

All of the other balls I find along the way still go in the bag.  Then I pull them out on my next range session and fire away.  I figure that TM or Cally ball is still better than the yellow rocks on the range.   This also removes the temptation to play a scrub ball when I fear the water.