How many injections does it take to numb a lower molar for extraction?

Sometimes one, sometimes more. Every extraction is a bit different. As long as you are anesthetized you won't feel a thing.

One or more. If tooth is abscessed and infection is spreading, more. If your nerve pathways are normal but unusually placed, more. If hard and/or soft tissue surgery is required, more. If you body reacts atypically to anesthetic, more. But usually 1 or just a little more. Your General Dentist or specialist Oral Surgeon can give you a better estimate based on your physiology.