I never track it, unless as Barter.
"You are low/out of ammo" is a MC Move, make them buy, activate their crap's downside, make Apocalypse World seem real, announce future badness, &c.
Make sure it follows from the fiction.
Also, where do you get your ammo from? is a great question to ask PCs.
If absolutely everyone at the table things it's neat / add verisimilitude / are gun fetishists, then you could track every bullet if you wanted too. That personally sounds tedious to me.