Buky stands second and writes b. Its name is the Slavonic word for letters or characters, a plural, and it is the second half of azbuka, the word the tradition uses for the alphabet itself. Alone among the opening letters it carries no numeral value at all.
That absence is not an oversight, and understanding it explains the numeral system of the whole alphabet. Cyrillic numeration is Greek numeration, taken over wholesale: alpha is 1, beta is 2, gamma is 3, and the Slavonic letters that correspond to Greek letters inherit the Greek numbers along with the Greek shapes. But Byzantine Greek by the ninth century pronounced beta as v, so the letter that looked like beta was needed for the Slavonic v, and it took the value 2 with it. Slavonic still had a b to write, and the sign made for it arrived after the numbers had already been allocated. There was no free slot.
So buky sits in the alphabet in second place and stands outside the number series entirely, and the same is true of every other letter added for a sound Greek did not have. A reader can therefore look at a Cyrillic letter, ask whether it carries a number, and get a reliable answer to a different question: whether the sign came in with the Greek uncial or was supplied afterwards for Slavonic.
The letter itself survives in every modern Cyrillic alphabet, still second, still writing b, still numberless in the rare modern contexts where the old numeration is used.