I have multiple swollen lymph nodes in neck and back of head. They never get bigger their just there. Biggest one is 2.25 cm. Cancer?

Possible. Any lymph node larger than 2cm warrants further evaluation. However, the longer the nodes have been prominent but unchanged, the lower the probability of cancer. Other factors of importance include your age, smoking history, overall health, family history, etc. I advise you seek out a formal consultation from your primary care doctor. A biopsy may be indicated.
Not Necessarily. I would see your doctor if this is the case. A variety of things can cause swollen lymph nodes and it is important to figure out if this could be something more serious.