How many balls are in your bag at the start of play?

Three. I play very early so I find a lot of balls on Monday AM from the weekend warriors.

It varies depending upon how long its been since I bought balls.  The pocket in my bag that I use for holding balls holds about 2 dozen and I just fill that up and buy more when I get down to 5 or less.

Depends on the course, but I usually have 3 or 4 new or gently used balls for my home course.  I usuall also have a dozen or so chipping practice balls that I could use in a pinch, but it's rare that I lose a ball on my home courses.  On unfamiliar or more difficult courses, I usually would have at least 6 playable balls.

~10 or so. i play with the balls i practice chipping with.

Now that I'm using a push cart and I'm not so concerned with the weight of my bag, I keep at least 2 sleeves of new balls, and a dozen or so other balls.

Around 10 usually 4 or 5 new and 4 or 5 from previous rounds.

I keep 9 new and 10-12 from prior rounds that I am still using.

I try to start a round with at least 8.

I don't think I've ever made it through a round without losing at least one....

3-4 as I like to travel light. If I loose one or two I grab some more at the turn. I never yet lost all my balls but I am sure I could borrow one, or I always find a few while playing even if I loose my own. In a tournament I carry a few more as I can't swap in a different ball.

Usually 12 new balls, though 1 goes in my left pocket and 2 in my right pocket on the first tee.  Sometimes there are some shag balls in the bag as well.

On average, I keep one dozen golf balls in the ole' bag for play and practice.  I try to keep them all the same for consistency, but it's not a hard and fast rule.

I usually have 2 sleeves of new ProV1x and then about 10-15 gently used balls I buy from a website. I've had a few rounds where I just can't stop losing balls so I'll switch to the used ones.

A dozen new balls, used to be 18, but now I lose less so I don't need as many.  Haven't made it through one round yet without losing one, so looking forward to that.

After reading a few other comments, I'll add:

I start each round with a new ball.

(I keep two or three new ones in the bag plus at least six 'experienced' balls --- ones I've played before or found on the course.)

If I lose the new ball (usually at hole #3 - slicing into a pond or hooking into trees/OB); otherwise at #5 - creeks run down both the left and right of the fairway and converge about 90 yds from the green), I make myself use old balls the rest of the way.  If I don't lose it, and it gets a bit worn, then I'll throw it in the 'used' pocket and play another new one.  ...just a little incentive to avoid hitting into water...

10-15. new and used. I will play my old ones first and eventually get to the new ones.

Yeah I play the crappy ones first too. If I don't feel comfortable playiing it when it counts (haha - like it ever really "counts") then I leave it on the teebox for another group. I pick up a ball here and there and probably get up to ~ 20 before thinning the herd.

There's usually 4 or 5 balls kept in my push cart compartment for regular play (one of which I'll put in my pocket) then in the golf bag there are two sleeves of new balls.

Of course there's the large zip lock bag in there that contains the balls I found.

yeah. same here. I usually find a ball or 2 every week. I haven't replenished my bag with new balls in a while now.