Three views of a stack of cubes on a 3 by 3 grid are drawn: front, right side and plan from above. What is the least number of cubes that matches all three?

The plan has 7 shaded squares, each needing at least one cube. Front view: the middle line must reach 3; side view: the front row must reach 3, so the front-middle column is 3 high (it is the only column in both). The side view also needs a 2-high column in the middle row, and the front view a 2-high column in both outer lines: one 2-high column in the middle-left square and one in the front-right square does it. Total $3+2+2+4\times 1=11$. (12 is the greatest.)