At the end of the increase row you should have 51sts.
(WS). P20. (M1. P3) 7 times. M1P. P2. 51 stitches.
Since there are 8 increases across the row, you have to start the row with 43sts. There must be a mistake earlier in your knitting if you only have 20sts.
Usually, you knit the cuff, do an increase row and continue with the mitten. I'm wondering why you have bound off sts?
What were the directions at the end of the cuff and before the increase row that you quoted?