2-3 should do it, 1 isn't enough. I like to use 3, just take really deep breaths of it and hold it as long as possible and then do another until all the gas is gone.
You should clean out the canister regularly... if it's dirty then little bits of frozen material can blast into your lungs, which is bad. Honestly I think using balloons from the whip cream canister is the best, that way you're not inhaling cold gas. Inhaling directly from the canister has started to feel bad to me.
I just began inhaling straight from the canister, seems better to me and everyone who seems to do a good bit of nitrous/actually know something just does it that way. I've never felt it be cold coming out of the canister into my lungs though, not even remotely, even when I inhale it right away after releasing it.
I think the cartridges are crap. I have done nitrous for many years and we always had tanks of medical grade N2O. That kind is food grade and is really just robbing you of oxygen with very little of the contents of what you get at the dentist. We have had food grade also, it's an imperfect world, and I have watched people flop around like a fish out of water more than I can count. I have seen people split their heads open bust out their teeth falling to the ground and would never describe the experience as psychedelic. Laughing gas yes, psychedelic no. I have seen mild mannered people turn into monsters it's called hippie crack for a reason. One is never enough and after too many you loose yourself and become obsessed trying to find that buzz that will never come back. Just my opinion and I will do it again, just sharing some facts for you.
I disagree with pretty much all of this. Except that yes, people get very fiendish.
I've only had medical grade at the dentist, but the cartridges seem to work might damn fine for everyone else. The argument that medical grade is worlds better kinda reminds me of how people say LSD was better "back in the day" when its still the same stuff just a different dose. I've had it at the dentist countless times and I can tell no difference (of course it is mixed with oxygen at the dentist though and a smaller dose).
That doesn't make sense, medical grade or not, its still robbing you of oxygen if you're not breathing any oxygen. Even IF food grade had a ton of crap in it, its robbing you oxygen just as much as 100% nitrous would be. Now, people going ham hyperventilating on a balloon is robbing you oxygen and will make you fall over and pass out, or worse.
I find it to be VERY psychedelic when combined with a psychedelic, its a complete wonder to me there doesn't seem to be any good explanation for it, but it blasts you the fuck off. A sober whippit to me is totally pointless and boring, definitely a novelty the first couple times though.