From what I understand, the Hebrew title “בְּמִדְבַּר” of the Book of Numbers literally translates to “In the Wilderness”. Why is it called Numbers instead?
According to the wikipedia article on the Torah, the Hebrew/Greek titles of the Torah are:
- Bəreshit (בְּרֵאשִׁית, literally "In the beginning")—Genesis, from Γένεσις (Génesis, "Creation")
- Shəmot (שְׁמוֹת, literally "Names")—Exodus, from Ἔξοδος (Éxodos, "Exit")
- Vayikra (וַיִּקְרָא, literally "And He called")—Leviticus, from Λευιτικόν (Leuitikón, "Relating to the Levites")
- Bəmidbar (בְּמִדְבַּר, literally "In the desert [of]")—Numbers, from Ἀριθμοί (Arithmoí, "Numbers")
- Dəvarim (דְּבָרִים, literally "Things" or "Words")—Deuteronomy, from Δευτερονόμιον (Deuteronómion, "Second-Law")
You'll notice that in English, none of the names by which we typically refer to the books of the Bible are incipits like they are in Hebrew. So I'm not sure why you singled out Numbers with your question.
As with the other names originating from the Septuagint, it's based on a topic of the book. as the Encyclopedia Britannica says
The English title is a translation of the Septuagint (Greek) title referring to the numbering of the tribes of Israel in chapters 1–4.