Hi Welcome to KH.
I think, how I read it, the 31 stitches is the pattern. If you want to repeat the pattern say 3 times you cast on 93 stitches plus any border stitches like 3-4 stitches each side for a gather stitch border. how many stitches you cast on depends on how thick your yarn is.
Are you on Ravelry, look up baby blankets and see what other people have done. or use www.knittingonthenet.com, there will be different patterns on there. I think for a DK/sportsweight like lion brand microspun you would cast on something in the region of 120 stitches.
Hope this helps, maybe somebody else will have more advice for you. show you progress and we all love baby pictures with lovely cuddly blankets.