I'm assuming your are starting round 20 with 42 stitches, yes?
If that's the case, you will decrease 6 stitches evenly across, so that means you would make a decrease every 7 stitches (42 divided by 6).
On the next row, you will have 36 stitches, so when you divide 36 by 6, you get 6. So you would make a decrease every 6 stitches.
And so on.