How many consonants did Hebrew have?
This is a trick question. The answer depends on what you mean by a consonant: spoken or written. Today everybody knows we count 22 letters in the Hebrew alphabet. But everybody knows, and forgets, that it has 23 consonants: original Śin is still with us. Sorry.
Writing is the only way we can learn what the ancients said, but writing is not language. And as writing reveals, it always conceals something of what it transmits.
There's no better example of this than the way writing masks the sounds of speech even as it immortalizes them. Since the 19th century, scholars have argued that there were actually 25 sounds in ancient Hebrew and Aramaic: it has been long noted--and best argued by Joshua Blau--that the Septuagint distinguished original ǵayin and ḥa in many place names and other transliterations. During the 80's, Richard Steiner first realized that there was an entire Aramean religious liturgy--really a kind of alternate-universe Hebrew Bible, including a pagan version of Psalm 20 with Baal instead of the Lord, and mourning for an Exile (with the Assyrians around, lots of people got exiled), transcribed into Demotic in Egypt, that distinguished these two consonants.
The Hebrew and Aramaic writing systems had concealed some of the most basic facts of Hebrew and Aramaic from us for almost 2,000 years. This has to do with their own histories--they're both derived from Phoenician, which lost those two sounds, along with original śin, sometime before the first Phoenician writing (11th century B.C.E., depending on what you mean by "Phoenician").
I stole the lousy Śin joke from Richard Steiner, who published the definitive treatment of all these issues. The answer to the question of when, and how, Hebrew went from having 25 consonants to 23 holds lessons for us about the relationship between writing and language, as well as for when different parts of the Septuagint were created.
Blau, Joshua, 1982. On Polyphony in Biblical Hebrew. Proceedings of the Israel Academy of Sciences and Humanities VI/2.
Steiner, Richard L. 2005. “On the dating of Hebrew Sound Changes (*ḫ>ḥ and *ǵ>ˁ) and Greek Translations (2 Esdras and Judith) JBL 124:229-67
--originally posted June 30, 2005, updated today.