A dozen new and 6 or so used. My course is very tight and I usually find 4-8 balls a round... but they are rarely Pro V1 or V1x class

I always have 7 ProV1's in my bag.   I mark 4 of them to use in that day's round.  However, I will play the same ball for at least 9 holes before I change or begin rotating them until I get my money's worth and put them in the shag bag.  

my bag is way too full of balls....I keep some balls I find, but they end up in my shag bag.... I only keep Titleist in my bag, any others I find get hit away.... I do keep a sleeve of Prov1's and Prov1x's  in my bag

I usually bring 2 sleeves, and that works for me.  I don't hit em far enough to lose them

I typically carry 6 to 8 new balls in my golf bag, but I do make exceptions.  I recently had the opportunity to go to the American Club in Kohler, WI with a friend for a long weekend.  I took 18 total balls with me.  We started in strong winds on the Meadows Course at Blackwolf Run.  I simply did not play well.  I lost 6 to 7 of the balls I had taken and I still had two days to play.  The next day we lined up to play the Straits Course in blistering winds of 25 to 35 mph.  If you've not seen the Straits Course in person it's hard to understand how intimidating it looks.  Given the wind, coupled with the way I'd played on Friday, and the difficulty of the Straits Course, I ultimately blinked and bought 6 more golf balls.  So what happens?  I played excellent golf all day, broke 90, and finished with the same Pro V1 that I started the day with.  Go figure.  In fact, I played the first 8 holes of the River Course at Blackwolf Run with the same golf ball the next day.  But as soon as I thought I might play the entire rounds with it, I promptly hit it into the proverbial river.

But what a great weekend!

Carry about a dozen in around, 3 are usually new, pro v1s tend to last between 3 and 4 rounds for me