Pattern repeat is 18 stitches + 1. Be aware that the number of stitches does not remain the same on every round. An accurate count of stitches may be taken on rows 12, 14, 26, 28. On rows 13 and 21 work sl1-k2tog-psso instead of ssk (in the end of a repeat), k2tog (at the beginning of next repeat) between two repeats.
Shown with a cast-on count of 19 stitches.