Microneedling head only had one needle: curious whether the treatment done on me was correct?

It sounds like you were treated with a machine that was designed for permanent makeup as opposed to micro-needling. Generally microneedling is performed with multiple needles. The technique can be used exclusively to treat problem areas only. However, a microneedling facial should involve treatment of the entire face and the neck if desired. 

Hello and thank you for your question.
The treatment you had done is called needling. With needling usually areas of the face are spot treated. Microneedling is done with a medical device with anywhere from 9-12 needles. A controlled injury is done to your full face and neck, or other areas of the body where you want to rejuvenate the skin. Most patients love that we can go right under the eye area and upper lid to help tighten, strengthen, and improve the aging skin.
Including PRP with your microneedling is always recommended.
Hope this helps,

microneedling is usually performed using multiple fine needles. The whole face can be done as well as other areas in your body. I would voice your question to the doctor who performed the procedure, although single problem areas can be done without treating the whole face.
Thank you for your question.