43 clues.
Sudoku problem :
+---+---+---+---+---+---+---+---+---+
|   | 8 |   | 3 |   |   | 7 |   | 6 |
+---+---+---+---+---+---+---+---+---+
| 6 |   | 3 | 2 |   | 8 | 4 |   |   |
+---+---+---+---+---+---+---+---+---+
| 2 |   |   | 4 | 9 | 6 | 3 | 1 |   |
+---+---+---+---+---+---+---+---+---+
| 8 |   |   |   | 6 | 4 |   |   |   |
+---+---+---+---+---+---+---+---+---+
| 7 |   |   | 1 |   |   |   |   |   |
+---+---+---+---+---+---+---+---+---+
|   | 9 | 6 |   |   | 7 |   | 4 | 2 |
+---+---+---+---+---+---+---+---+---+
| 3 |   | 8 |   | 2 |   | 1 | 5 | 4 |
+---+---+---+---+---+---+---+---+---+
| 4 |   | 9 | 8 |   | 1 | 6 |   |   |
+---+---+---+---+---+---+---+---+---+
| 5 | 6 |   | 7 | 4 | 3 | 2 |   | 9 |
+---+---+---+---+---+---+---+---+---+

Solved Sudoku :
+---+---+---+---+---+---+---+---+---+
| 9 | 8 | 4 | 3 | 1 | 5 | 7 | 2 | 6 |
+---+---+---+---+---+---+---+---+---+
| 6 | 1 | 3 | 2 | 7 | 8 | 4 | 9 | 5 |
+---+---+---+---+---+---+---+---+---+
| 2 | 5 | 7 | 4 | 9 | 6 | 3 | 1 | 8 |
+---+---+---+---+---+---+---+---+---+
| 8 | 3 | 2 | 9 | 6 | 4 | 5 | 7 | 1 |
+---+---+---+---+---+---+---+---+---+
| 7 | 4 | 5 | 1 | 8 | 2 | 9 | 6 | 3 |
+---+---+---+---+---+---+---+---+---+
| 1 | 9 | 6 | 5 | 3 | 7 | 8 | 4 | 2 |
+---+---+---+---+---+---+---+---+---+
| 3 | 7 | 8 | 6 | 2 | 9 | 1 | 5 | 4 |
+---+---+---+---+---+---+---+---+---+
| 4 | 2 | 9 | 8 | 5 | 1 | 6 | 3 | 7 |
+---+---+---+---+---+---+---+---+---+
| 5 | 6 | 1 | 7 | 4 | 3 | 2 | 8 | 9 |
+---+---+---+---+---+---+---+---+---+

Recursive calls from solve() = 164