How many missions does the Starcraft 2 campaign have?

The campaign lists 26 total missions, though some of these are "A or B" missions. Though you may only complete one of these missions for credits / research / campaign credit, the other, alternate mission is available on the console on the bridge to be replayed any time you like for achievements, etc.

Given the nature of those "pick one" missions, I believe there are 29 total possible missions +- a bonus mission that is only unlocked during special circumstances. (Or there may be 26 missions +- the bonus mission, I'm unsure how the "Campaign progress" bar, x/26, accounts for the branching scenarios.)

25, 1 bonus, and 3 alternates

25 + 1 bonus is the "26" listed at the bottom of the screen.

There are a total of 29.