Mishnah Berurah — משנה ברורה

It is invalid - Because he transgressed on [the prohibition of] "Do not add". And the Gr"a in his commentary agreed that the law is like the opinion of the Itur which is brought in the Beit Yosef, who holds that there is no [prohibition] of "Do not add" here. And this that the Talmud says it's invalid [when done] like this, that's specifically when one added [a string] from a different species to the [other] strings [for example strings of cannabis or cotton wool]. But [if one added another string] from the same species, they permit it, even optimally, to add as much as he wants. And see in Artzot Hachaim, where he brings a few more from the great early rabbinic authorities who hold the same way. And therefore, we conclude that if he already made them [with extra strings] and it's impossible for him to fix them, he may wear them. But as soon as he is able to fix them, he must fix them. But [regarding] lessening from [the number of] the strings, everyone agrees that it is invalid, as is seen in Beit Yosef.