Uk stands twenty first and writes the vowel u. It is not a single sign in the early alphabet but a digraph, written as the letter for o followed by the sign taken from Greek upsilon, and its name is the Slavonic word for teaching or learning.
The compound spelling is a Greek inheritance. Greek wrote u with omicron and upsilon together, and the Slavonic translators took the convention with the letters, even though Slavonic u is a plain vowel needing no such treatment. It is one of the clearest instances of the alphabet importing a Greek orthographic habit that its own language did not require.
Scribes soon economised. The two elements were stacked one above the other, and the stacked form eventually became a letter in its own right, which is the ancestor of the modern printed u in the East Slavic alphabets. Both the wide and the stacked forms appear in the church books, distributed by position rather than by sound.
The numeral value is 400, and here the tradition divides against itself. The manuscript tradition gives 400 to uk; the printed Church Slavonic tables give it to izhitsa. Both usages are attested, they are not reconcilable, and a reader working with a number in the four hundreds needs to know which tradition the book in front of them follows.