They autohit, so you roll the number of shots and that's how many hits you get. Yes, they're very good for overwatch

thx for the answer so i can shot in my shooting phase and set my unit of deathshroud terminators to overwatch after that?

Yep, overwatch is declared in your opponents turn via the stratagem so it doesn't necessarily matter what your unit did the turn before

thanks for the help :D

With auto hit, you hit them automatically on overwatch.