It's easy to see that the answer cannot be more than 8 (since there are only 8 rows and columns on a chessboard).
It's less easy to see that the answer is exactly 8. There are several possible solutions. One simple one is the "staircase" solution - verifiable as an existence proof.
Place queens on
An official chessboard is an 
board containing squares alternating in color.
We can't have more than 8, since you would have a row (and column) with more than one queen on it.
There are 12 unique solutions, plus an additional 80. See the image.
12 Possible casase
The 12 distinct solutions for 
are illustrated above, and the remaining 80 are generated by rotation and reflection.
Read more:
http://mathworld.wolfram.com/QueensProblem.html
