I know some folks who run a box of 50 FMJ and call things all good. . .
There's no one-size-fits-all answer for this.
How often do you practice with that gun?
Do you always practice with your HD/SD ammo, or only use cheaper range ammo?
Do you have mag/s for HD, and others for range practice?
If you practice often, then maybe 100-200 rounds of defense ammo is sufficient for an initial function test. After that, you should continue to fire that ammo at each practice, at least 50-100 rounds or more.
That'll keep your ammo rotated. The worst thing you can do is leave your defense ammo at home, shoot bulk ammo at the range, then load the same old HD ammo in the same untested mag for protection. Refresh your ammo at least every few months, not only to have fresh ammo, but to continue making sure your pistol and magazine is functioning perfectly with it by using up the old stock.
If you use a revolver, then mags are not an issue. Otherwise, I have range mags that get scuffed and dirty, and HD mags that only see bullseye practice time. My 2 HD mags run HD ammo exclusively. I always have a spare mag loaded next to the pistol for malfunction/reload. Two is one -- one is none.
If you have any issues with your chosen ammo cycling or firing, do some troubleshooting. Could be the gun needs cleaning, might be a mag issue, or you might want to test other brands/versions of HD ammo. Once you think you have the issue corrected. Restart the testing by running 100-200 rounds through it again. If you get 4 boxes to run cleanly start to finish without any issues, I'd trust that ammo.