The Shulchan Aruch writes that the custom regarding the string wrappings of the tzitis is for the first wrapping to be of 7 windings, the last wrapping of 13 windings and the two middle wrappings should be of 9 and 11 wrappings respectively. The gemora Menachos (39a) writes that one should not make less than 7 or more than 13 wrappings. The 7 corresponding to the seven heavens (רקעים) and the 13 to the 7 heavens plus the addition of the spaces between each heaven (7+6). In a fascinating paragraph, the Chasam Sofer (שו"ת או"ח עב ד"ה והנלע"ד) explains why we see in certain instances that a number corresponds to something else, as in our case, and one shouldn’t exceed that number, yet in other cases, for example the two Shabbos candles that correspond to זכור and שמור, one is allowed to add additional candles. In short, he explains that if what is being done is something that we would have done even without Chazal telling us, e.g. lighting candles for Shabbos, then one is allowed to add additional candles. However, if we would have never thought of doing it on our own, e.g. wrappings by tzitzis, then we should remain constricted by Chazal’s suggested number. (It is important to note that b'dieved the number of wrappings does not negate one’s fulfillment of the mitzvah.)
The Mishna Berurah writes that in a situation where one needs to cut his tzitzis strings, a (metal) knife should not be used. One should rather cut them with his teeth. The reason for this halacha is that since the mitzvah of tzitzis has the zechus and ability to give one length of days, it is improper for it to come in contact with a metal knife, which can be used as a weapon to shorten one’s days. This is an extention of the mitzvah in the Torah (Devarim 27:5) not to use iron in the construction of the mizbeach. The Shelah writes that there are 32 strings and one has 32 teeth. This relates to the 32 pathways of knowledge.
