The directions appear okay, though a bit basic.
You're right, it does not say how many buttonholes to place, just to place them about every two inches. The Ravelry photo shows 5.
The collar (neck band) is made first. The first button hole is placed on the collar. From there you switch from garter stitch to stockinette (except for the button bands). You'll place markers for the raglan sleeves, work some increases and eventually put the sleeve stitches on holders. You will continue on with the body of the sweater to the hem where you once again put a garter stitch border/hem. Then go back and work the sleeves.
The body is worked flat.
The sleeves are worked in the round.