Depends on a lot of factors. Hypoxia does worse damage the longer it persists. It can cause death, permanent brain damage, recoverable brain injury, or a headache. If you're using canisters, not holding your breath, breathing air between canisters, and not doing it excessively, hypoxia won't be an issue. You can't die from a dispenser. If you're breathing multiple canisters in a row with no breaths between, maybe don't do that. You're not gonna die, nor will a serious injury occur. But it's likely not good for you.
If you're using a big tank, yes you can kill yourself or damage your brain irreversibly.
But what if youre doing say 20 canisters in a row and the only break you have in between is the 2minutes it takes to inflate you and your friend's next balloon?