Two of the seven dwarfs have names beginning with S: Sleepy and Sneezy.
Disney's seven dwarfs are Doc, Grumpy, Happy, Sleepy, Bashful, Sneezy, and Dopey. Counting only the first letter of each name gives two S-names: Sleepy, the perpetually drowsy dwarf, and Sneezy, whose powerful hay-fever sneezes repeatedly disrupt the cottage.
The question refers to the named dwarfs in Disney's 1937 film, not to every version of the Snow White story. The Brothers Grimm's original tale did not give the seven dwarfs individual names, so there would be no canonical set of initials to count there.
A useful memory aid is that the names divide into two D-names, Doc and Dopey; two S-names, Sleepy and Sneezy; and three remaining personality-based names, Happy, Bashful, and Grumpy. The two S-names are easy to confuse because both characters are frequently shown near one another in group scenes.