Not sure if this is the best place for this question but here it goes:

why do people mark/number their magazines?  is it just to keep track of each one in case "#3 failed at the range today" so you know which one sucks or what?  

as always thanks for the info

Yes, I think your assumption is correct. I've had a magazine fall at the range before and accidentally put it back into the mix with all my other magazines and couldn't tell which one was the bad magazine when I got home.

Only reason I did it.

I just step on the bad ones right there and then.....

Another reason is that you could be at a shoot with a bunch of guys who all brought identical mags.

To keep track of failures and to tell my mags from everyone else's.

I only do it so I can rotate which mage are loaded / getting used.  

I will periodically unload all my loaded magazines and load the next number down the line.  Probably more OCD than necessary, but I like knowing there isn't one magazine that sits around loaded indefinitely, or more than the others that I keep in the rotation.

For each firearm that gets used regularly, I have at least 10 magazines in rotation that get numbered.  The ones I store don't get numbers, since they aren't in the rotation.  I tend to leave one magazine loaded, and I take five to the range.  Numbering just helps me keep the rotation evenly distributed.

That's my general rule for pistols, my AR and AK mags don't get numbers since all the ones in rotation are loaded and go to the range all the time.  If one fails, I focus on that one for the rest of the range session.  If it continues to fail, I either remove it from circulation, or try to figure out what is wrong.

Numbering also helps me do a quicker inventory.  Instead of counting them and trying to remember how many I had last time, I can just look for a missing number.

I have dedicated magazines for range practice only that are marked with a number sticker on the bottom.  I do this only to keep track of the number of rounds I shoot and to keep these magazines out of my EDC group.

 
 
 

Yep, keep track of training vs life-on-line mags.

Paint pen works wonders.