What I meant by it is buy to replenish your ammo supply where you think it’s a good value to you…
Which is also part of
is a ideal amount to have
And, that is going to vary.
In fairness, the last case buy of ammo I made was like in 2002, maybe '03--and that was entirely based on the selling price. (From potentially hazy recollection, it was WWB 5.56 55gr FMJ or WWB FMJ 9x19, and the price was at or under $3/bx.) The rest of the time, ammo was either reloaded, or bought a box or two at a time.
Some of that was a habit of "paying rent" for hanging out at the LGS by buying a box or two of ammo. Hang out once a week at the LGS, and you wind up with 50 or 100 boxes (less whatever was sent down range). This is good to keep the light on in the LGS, but, also keeps a person very much in close touch with on-the-shelf prices, too. Which is handy if you then see bulk prices elsewhere.
Now, is it "better" to only suffer pricing at one box at a time, or take the hit 1000 rounds at a time? Dunno.
For me, I can find room for two more boxes of ammo easier than an entire case.
Ok, one downside to my method is that I have random bits of ammo. An, worse, partial boxes of rando ammo. Like revolver ammo for me, as it's been a decade since I had a revolver at all. Or battle-packs of 308. Or the six rounds of homeless 25-06. So, I have 600% more .257 ammo than I (presently) need. But, say the LGS gets a 25-06 in on the used rack for a decent price. Then, I'd be in need for several boxes of ammo.
I'm passing certain I have a couple of cases of 45acp, all different brands and loadings of FMJ. Is that "enough"? Dunno, never "feels" like it. Even if US$27/bx discourages more than a little. I have like elebenty cases of 5.56x45, did not stop me from getting some generic Armscor at the last gun show (it was only US$10 the box, and from a guy out of Brenham that turns out to have run in many of my Central Texas circles).