Shta stands twenty eighth and writes the cluster sht. It carries no numeral value.
It is unusual in what it represents. Most letters write a single sound; this one writes a sequence of two, and the sequence it writes was the regular Old Church Slavonic outcome of certain inherited consonant groups. Giving that outcome a letter of its own is a decision about the language as much as about the script: it treats a cluster as a unit because the grammar treats it as one, appearing where a related word has a different consonant entirely.
The shape is built on sha. Whether it is a ligature assembled inside Cyrillic from sha and tverdo, or the Glagolitic sign for the same cluster redrawn to match the Cyrillic sha, is genuinely argued, and the argument is recorded on the relation to the Glagolitic letter.
Its later history is the most divided of any letter in the alphabet, because the cluster it names developed differently in different Slavic languages while the letter stayed put. What a Bulgarian reads and what a Russian reads are not the same sequence of sounds, and the companion article takes that up. In both languages, and in several others, the letter is still in daily use.
It is also one of the small number of Cyrillic letters that descend below the baseline. That is a typographic fact with practical consequences: a script whose letters have descenders needs more room between lines than one whose letters do not, and Cyrillic type is set more loosely than Greek partly for this reason. In the earliest hands the lower element sat closer to the body of the letter, and its migration downward and to the right happened gradually across the manuscript centuries before the first printers fixed it in metal.