Yes, human organs don't like hypoxia. A great care should be taken if you ever had pneumothorax or heart diseases.
I don't know about certain limits. 5 - 10 experiments should be enough to figure out how it works. I think, it makes sense to make 10 - 15 minute or longer breaks between them. I never carried out more than 3 tests per day, and I had no any noticeable adverse effects so far.
For treatment of depressive mood, I decided to use at most 3 chargers per session within one day, with intervals of at least one month between the sessions (in order to avoid addiction and B12 deficiency).
Some sources claim that producing hypoxia with nitrous is never safe and in rare cases people die because of heart failure. I didn't see any details revealed there though, so maybe that's just fearmongering.
I guess, you need to be sober in order to understand the procedure clearly and carry out it properly. Some amount of N2O may be used beforehand for coping with anxiety or stress.