Well
Not trying to be a smartass, but at least two.
The whole point of a vertical tasting is that you're trying to get a sense of how the same beer recipe ages over time. For that, you need two different batches, or one batch aged under different conditions (e.g., one kept at 32 F for a month, one kept at room temperature).
For big beers which develop slowly, it's more useful and more fun to have a flight of at least 3 to 5, though.