an even number.
Slip stitch into the second chain from hook. (Slip stitch is inserting your hook, yarning over once and pull through both loops) Then half double crochet (HDC) into the next stitch. Alternate a slip stitch (SL ST) with HDC to the end of your row. The last stitch should be a SL ST. Chain one and turn.
SL ST into the SL ST you just made then HDC into the HDC. Alternate again all the way across your row. Always SL ST into a SL ST, always HDC into a HDC. Always chain one and turn.
See video below to watch a demonstration.