Given is a chessboard 8x8. We have to place $n$ black queens and $n$ white queens, so that no two queens attack. Find the maximal possible $n$. 
(Two queens attack each other when they have different colors. The queens of the same color don't attack each other)
To solve this problem, we need to place $n$ black queens and $n$ white queens on an 8x8 chessboard such that no two queens attack each other. Queens attack each other if they are on the same row, column, or diagonal. However, queens of the same color do not attack each other.

1. **Understanding the Constraints:**
   - Each black queen must not attack any white queen.
   - Queens of the same color do not attack each other.
   - The maximum number of queens of each color that can be placed on the board without attacking each other is 8 (one per row and column).

2. **Initial Consideration:**
   - If we place one black queen and one white queen in each row and column, we need to ensure they do not attack each other diagonally.

3. **Placing the Queens:**
   - Consider placing black queens on the main diagonal: $(1,1), (2,2), \ldots, (8,8)$.
   - To place white queens, we need to ensure they do not share the same row, column, or diagonal with any black queen.

4. **Maximizing $n$:**
   - If we place black queens on the main diagonal, we can place white queens on the anti-diagonal: $(1,8), (2,7), \ldots, (8,1)$.
   - This ensures that no black queen attacks any white queen, as they are not in the same row, column, or diagonal.

5. **Verification:**
   - Black queens are placed at $(i,i)$ for $i = 1, 2, \ldots, 8$.
   - White queens are placed at $(i, 9-i)$ for $i = 1, 2, \ldots, 8$.
   - No two queens of different colors share the same row, column, or diagonal.

Thus, the maximal possible $n$ is 8.

The final answer is $\boxed{8}$.