##Rashi and Tosfot hold that two strings out of eighth are techelet. [https://www.yutorah.org/sidebar/lecture.cfm/788501/rabbi-hershel-schachter/hilchos-tzitzis/ Rav Schachter ("Hilchos Tzitzis" beginning)] holds like Rashi and Tosfot and recommends all Ashkenazim to follow this method due to the plethora of sources advocating it (Baal haIttur, Tur, Shulchan Aruch, etc.).
##Rambam holds that one string out of eighth are techelet. Sefer Hachinuch, Maggid Meisharim, Mabit, Maharam Ibn Gabay, Sefer Charedim, Levush, Gra, Sefer Hakaneh, and Arizal agree with the Rambam. Radziner in Petil Techelet ch. 6 ends up following the Rambam but says one has what to rely upon to follow either of the other opinions. Ben Ish Chai (Noach n. 17) and in [http://www.hebrewbooks.org/pdfpager.aspx?req=41237&st=&pgnum=101 Od Yosef Chay (Vaera n. 3)] holds like the Rambam. [https://www.ykr.org.il/question/1629 Rav Yonatan Mazuz] agrees.