On average, how many numbers are given in a Sudoku puzzle?

It's difficult to answer that one because it so strongly depends on the difficulty of the puzzle. As a rule, the puzzlemaker must give enough numbers so that there is only one solution to the puzzle. One of my Sudoku books contains puzzles of all difficulty levels. The easy puzzles contain 35-45 given numbers, while the hardest all contain about 25-26.

I seem to remember that the minimum number of givens necessary to ensure a unique solution being one of the unsolved problems of recreational maths.
It probably says something about that here:
https://en.wikipedia.org/wiki/Sudoku