To knit stockinette stitch you can cast-on any number if stitches that yoy would like.
Row 1: knit all stitches
Row 2: Purl all stitches
Continue repeating rows 1 and 2 until desired length. FYI: Stockinette stitch curls so you will want to have either a knit stitch border or even rib stitch. This is 10 cast on stitches 10 rows.