What is the maximum number of Sudoku boards possible?

Question

Sudoku is a $9 \times 9$ board game subjected to $4$ conditions:

1. Only digits ($1-9$) can be used to fill a blank box.

2. All horizontal rows to be filled with digits without repeating a digit in row.

3. All vertical columns to be filled with digits without repeating a digit in a column.

4. All $3\times 3$ boxes to be filled with digits without repeating a digit in a box.

Let assume the other condition: every Sudoku board has a unique solution. There is no restriction on the number of filled spaces and blank spaces.

Subject to these conditions, how many permutations of Sudoku boards are possible?