Question on knitting pattern instructions

I have just started knitting and I am stuck on how many stitches to cast on. The book I am using for my pattern says Aghan and then it says Cast on 138 sts (stitches). Do I actually cast on 138 stitches? It says to use stockinette stitching and to use (which is what I am using) a 29" circular knitting needle size 11. The gauge is 14 sts and 18 rows = 4".

Yes, you cast on 138 sts, that would make it about 40 inches wide. If you use a slipknot, that counts as the first st and you’d do another 137.

Yep - you actually cast on 138 stitches.

You want to end up with 138 stitches to work. As Sue said if you use a slip knot that loop counts as the first stitch. Just be sure to end up with 138 loops on your needle to knit over to make the afghan.