The pattern stitch instructions give it as a multiple of 18 plus 5. I'm not sure how they figure that.
In order to start row 1 you need to cast on 7sts for the beginning + 18sts for the repeat + 12sts for the end (37sts total). So I would think 7sts + some multiple of 18 + 12sts.
I'd try it out on 2 repeats of the pattern just to see how that works out.
Sounds like you have your hands full!