Fuji Ito is one of the major brands for shamisen strings.
The middle string (ni no ito) offered here is made from silk, the traditional material used for shamisen strings. The thread size is 14, which baiscally works with any style. Nagauta players might choose a thinner string for the middle string, but it really is up to your own taste.
Please note that all strings are sold individually and not in multi-packs.