I try to keep the ball count as low as possible simply because I enjoy walking and having a lighter bag. Half and half - NEW and old. If I find balls on the course, I usually like to keep them so that I can hit them at the cottage ;)

Right now I have two new sleeves of ProV1x balls but have probably have 8-10 old balls.  Not sure why I carry that many, but that's what I usually carry in my bag.

I keep 4 to 6 in my bag and they are a mix of old and new, depending if the old ones are beat up or not.  I keep found ball so if they are ProV1's or NXT Tours and are in good shape.  Otherwise I leave them for the cart boys.

A pouch of about 12 Pro V1/x and another with about 10 or so "water/waste" balls that I can hit. Usually the ones I find on course

Usually a few sleeves of ProV1X's.  :-)

I normally carry 6 new balls and 6 referbish use referbish holes tough Hazzards 

I normally carry 8-12 new balls and 4-5 "junk" balls to practice second shots

I have a box of ProV1Xs. Once the box is 2 sleeves or less I buy another at the pro shop. Any scuffs from a cart path, I change the ball and add the damaged ball to my practice bucket. I've got a huge pet peeve with picking up balls on the course in fear that I may be picking up someone's wayward shot. I can't stand ball hawks in the line of play on any part of the hole. I know I've lost some strokes from someone picking up or playing my ball. Something I've picked up from my dad. If you know it's not your ball, just leave it and move on to your ball...

Probably around 10 at any given time. I carry warm weather and cold weather balls. 

About a dozen, even though it is probably too many. Mixture of new and old. I remove the worn ones to hit into the net in my basement.

Maybe 2-3 new sleeves, and 3 or 4 carryovers from a previous round.  Any found balls are pulled before the next round.  ProV and NXT balls go into the shag bag, others - I typically  store them up and then when I'm bored, I'll practice my hybrids or driver by blasting them into the woods that border my home.

I start out with 2 sleeves of new balls and a few previously used balls.  I play the same ball for 6 holes unless in on a good streak and then I just keep playing the same ball.  My ball of choice is the ProV1x but I keep a couple ProV1's in the bag to try different shots.

Somebody convinced me to try and play a round with just one ball (not sure, think it was inspired by a Nicklaus story) so if you lost it, the round was over.  It was actually super-fun.  It's a whole round of nerves and adrenaline, knowing if you lose it, your done for the day and out $50 for the round....

Not only was it a blast, was actually a bit useful too, as it taught me a few things about my game.  

I did finish, and my score worked out to be about the same.  Obviously forces you to be way more conservative than normal..... which helped on some holes and hurt on others.

Would highly recommend trying it once.

I no longer carry very often so usually load the bag with a dozen Pro V1x and have 3 or 4 in gaming circulation that I'll rotate to the shag for practice once marked up.   Not usually less than 6 though as I have 5 ball drills I like to warm up with.

If I walk and carry (which I do now and then to change it up, or to enjoy a particularly quiet, still evening round), I dump everything and anything not needed - including certain clubs and balls

Don't usually pick balls up - If it's a Pro V1 or Pro V1x in decent shape it'll go into the shag.

I usually have around 10-15 balls in my bag, mostly balls I've used on previous rounds and a sleeve of new DT Solos almost every time.

I keep at least 2 new sleeves of ProV1x and 3-4 used ProV1x and some NXT Tour s Balls from previous rounds in my playing pocket.  I also keep 3-5 practice ProV1x / Test balls for short game work in a separate pocket.

Normally I only carry about a dozen or so. In a tournament of big round I will carry three new sleeves and nothing else. When I am playing for fun, I like to carry two sleeves of new ones and 6 or so used ones. I also pick up any balls I find on the course. Obiously only if they're Titleist :)