Yer stands twenty ninth and writes the back jer, one of the two reduced vowels of Old Church Slavonic. It carries no numeral value.
A reduced vowel was a real vowel, short and weak, and in the earliest texts it is written wherever it was spoken, which is very often: Slavonic words could not end in a consonant, so almost every word ending in what a modern speaker hears as a consonant carried one of these letters at the end.
Two facts about the jers are worth holding together. They were pronounced, at least at the beginning, and the manuscripts are consistent enough about writing them to show it. And they were also doing a second job, telling a reader about the consonant before them: a back jer implied an unsoftened consonant and a front jer a softened one.
The first of those functions disappeared. The second did not, and it is the whole reason the letter survives in the modern alphabets as the hard sign, a letter with no sound of its own whose only work is to say something about its neighbour.
The sound change that produced that outcome is the most studied event in Slavic historical phonology, and it is set out in the companion article. Its consequences reach into every Slavic language spoken today.