How common is it to be knocked out with laughing gas at the dentist?

Yes and no: Usually knocked out word is an expression of direct or indirect trauma. Nitrous oxide sedation (laughing gas) is not an injury. Good luck.

Nitrous Oxide: It's a inhalational anesthetic and analgesic . Very commonly use with dentist. Should not knock you out completely when use by itself. Will kill pain and induce a pleasurable feeling if well being. Feeling of euphoria, floating. That will be an adequate level.

Maybe: People react differently to various sedation techniques. Nitrous oxide is a good tool to use for reducing anxiety level and help patients to tolerate the procedures better. Some feel more sedated than others.

Laughing Gas: Also known as 'sweet air' or n20\o2 analgesia is commonly used by dentists. Sometimes it's used instead of local anesthesia to relax patients and take the edge off, but more frequently is used in conjunction with local anesthesia. As used in the way, it relaxes you, dulls your senses and makes the entire visit easier.