Does allergy testing detect all your allergies or just a few?

Wrong question. A better approach is to state your symptoms. Then your dr can begin to help define & then relieve these symptoms. Allergy testing is of very limited use towards finding the best treatment.
Many allergens. Allergy testing can detect: allergies to foods, environmental (pollens of trees, grass, and weeds, dust mite, animal dander, cockroach, fungus), penicillin, local anesthetics, venom (bee, yellow jacket, hornet, wasp, fire ant), and latex. Patch testing can check nickel, gold, fragrance, multiple chemicals etc unfortunately, ige testing is not available for most medications, and dyes/preservatives.