You only need enough torque to hold the board in place without it wiggling.
To attach your motherboard to the case, screw it in. First, seat the screws and give them a couple of precursory turns. Then, proceed in a star pattern, tightening each screw a little at a time. Don’t go wild while tightening, as you might damage the board. You only need enough torque to hold the board in place without it wiggling.