Now a days a beged with tzitzis has on each corner 4 strings that are then tied in such a way so it comes out as 8 strings altogether hanging down. It is then connected to the beged with 5 knots and a number of times wrapped around between each knot (with various minhagim on how to tie the tzitzis exactly.)
This is what we do now, however m’doraisa what does a beged with tzitzis look like? How many strings would satisfy the requirement m’odoraisa? Would one string (tied in a single knot — kesher elyon d’oraisa (the top kesher in min hatorah) and hanging down as 2 fulfill it? Do we need 4 (doubled to 8) strings m’doraisa?
The Rambam in the begining of hilchos tzitzis writes that the number of strings is not of Biblical origin, see the Kesef Mishna there.