Initial izhe is a second form of the letter izhe, and the Unicode Standard gives it a code point of its own under that name. It is a positional variant: a shape used at the beginning of a word where the ordinary form is used elsewhere, on the same principle by which several scripts distinguish an initial from a medial letter without treating the two as different letters.
Variants of this kind are the reason the number of letters in Glagolitic is quoted differently by different authorities. A count that lists every distinct sign attested in the manuscripts arrives at a larger figure than a count that lists only the letters of the alphabet as the abecedaria recite them, and both counts are defensible because they are answering different questions. The recited alphabet has a fixed length; the repertoire a scribe actually used does not.
Being a variant, the sign carries no numeral value of its own. The count of the alphabet runs through the ordinary form of izhe at ten and passes over the variant entirely, which is consistent with the treatment of every other positional or regional form in the script.
Its presence in a text is a fact about the scribe rather than about the language. Nothing in Old Church Slavonic distinguishes an initial i from any other i, so the choice is one of hand and training.