Row 15: work 10 sts in purl, change to main color, work 20 sts st st. Change to cc work 5 st st. *repeat from * to * 5 times.
Yes, 14 rows in garter st (knit every row).
On row 15, you're setting up the patterns for some number of subsequent rows. You're right that stockinette is a series of rows alternating knit one row, purl one row. So one way to do row 15 is purl 10sts, knit 20sts in main color, change to cc and knit 5sts and do the repeat. On the next row, the stockinette knit sts should be purled.