You would add 1 stitch on each side for seaming purposes, and don't forget that when knitting in the round, every row (round) is a right side row so when you transfer that to knitting flat, every second row will need to be done in reverse to take into account the fact that you will be turning your work after every row.
Just looking in a knitting book I have that describes 'Purse Stitch' and the instructions it gives are:
Use any multiple of 2 stitches (plus don't forget your 1 stitch on either side that you're adding for seaming)
1st row: P1 *yo, p2tog; rep from * to last stitch, P1
Repeat this row
Looks pretty straight forward. Might not give the exact same look but it's definitely do-able.