The gauge for this pattern is not given in the book, but it is based on the Very Swedish Sweater (the pattern picture for the adult size sweater shows the cap on an adult) and the Very Swedish Sweater for Children. The sweaters have gauge at 18 stitches/4” on the larger needle, but the cap is knitted on the smaller needle, so guessing 20 stitches/4”.

This can easily be sized up for an adult, provided the stitch count is a multiple of two.