How often is general anesthesia needed for wisdom teeth removal?

Not often. It is more common to use conscious sedation and local anesthesia, or in many cases just local anesthesia. There are usually many factors that would be involved to warrant general for that procedure.
Varies. The manner in which any extraction is done is a decision made by the patient and the dentist. Some factors to be considered are the difficulty of the extraction, the general health of the patient, whether an infection is present, and the type of the patient in question.
Talk to Dentist. Most teeth extractions can be done under local anesthesia with a cooperative patient. If there are especially impacted teeth then some sedation might be indicated. I don't even remember the procedure after a little dose of valium. If you have concerns please contact your dentist and discuss these fears with him. Good luck.
Anesthesia. Rarely. General anesthesia may be used when removing wisdom teeth associated with complications and extensive dental procedures. Severely anxious and intellectually challenged individuals need general anesthesia.