I have 1 horizontally impacted wisdom tooth. Will I need to get rid of all of them or just that one?

Impacted toothq. This is impossible to say without an examination of your mouth and seeing the x-rays. It depends on where the wisdom are located, their angulation, how close they are to other teeth and nerves, how much bone is covering them, etc. Ask your dentist or oral surgeon.
Evaluation. Each wisdom tooth is evaluated on multiple factors. See a board certified oral surgeon for the best advice about wisdom teeh.