The number of strands and length of the strands depends on the size of the afghan o9r scarf and the weight of the yarn....and what you want it to look like.
I wind the yarn around a piece of cardboard cut to the size I want the fringe to be cut....wind enough for one "tassel"...cut it once, double it and using a crochet hook, pull it through a couple of loops of yarn on the bottom or edge. Then pull the loose ends through the loops.
Go to a store....look at afghans and scarves....see what you like and copy it.
Linda C

i have an old address book which seems to be the perfect width for the fringes that I've made.I usually use two strands together,which makes four strands when hooked through with the crochet hook.
I then lay the item flat after they're all on and trim the edges so that they are all the same length.
Eliza ann

I also knot the very tip of each acrylic yarn fringe so that it will not unwind in the wash.

I think there is a difference between fringe and tassels.

Fringe is just loose strings. Tassels are more strands (fatter) and are wrapped a short distance from the top so they look like, well.....a tassel!