Depends when, what, and why you vaccinate.
WEE, EEE, and VEE vaccines last six months. However, most of us don't live in places where bugs live year round. So most of us, if we vaccinate in early spring, can get by easily and safely with only one vaccine. If you live in the Everglades or rain forest, you'll need to do these shots twice a year.
Flu, Rhino, and most WN shots last TWO MONTHS. They need to be boostered many times a year for active traveling horses or horses at high risk. I use an annual flu (Recombitek) and annual WN (Previnile or Recombitek) vaccine to minimize # of vaccinations and cost. I still have to booster the rhino shots every two to three months for at-risk horses.
Tetnus and Rabies are annual shots. Tetnus vaccine has been shown to improve immune responce to other vaccines and is often times given multiple times a year without any harm.
Botulism is an annual shot after boosters have been done (three).
Strangles is an annual IN vaccine.
EVA is an annual shot.
That is most of them... there aren't too many other things you can vaccinate for other than Potomac, and I don't know anything about it.
So do I do more than two sets of shot a year? FOR SURE!!! But I do them only as often as they are needed to be effective, and don't overlap unless I have to.