The numbers between the arrows that are marked before and after the pattern parts show the number of stitches between the centre stitch and edge stitches.
The hearts pattern repeat is 20 stitches.
I suggest to use the garter stitch for last four rows as marked in the pattern. Otherwise the edge of the scarf may start curl up. Actually the whole edge lace can be knitted garter stitch. This is very much a question of taste. The thinner the yarn the less it plays a significant role. So one can make the edge how it feels better and looks better.