The Old Testament is also called ‘Ta•nach.’ In Hebrew it is not a name but the initials of the three sections of the Old Testament: Torah (Torah, the Law); ‘Ne•vi•eem’ (Prophets); and Ke•tu•vim (Writings). The Torah includes the five books of Moses (Pentateuch), and ‘Be•mid•bar’ (Numbers) is its fourth book, following ‘Be•re•sheet’ (Genesis); ‘Shemot’ (Exodus); and ‘Vaikra’ (Leviticus).
Each of the Torah books is named after either the very first word in that book (as in Genesis where the very first word ‘be•re•sheet’ is the name of the book), or the second (as in Exodus where it is the second word in the verse), with one exception occurring in the book Be•mid•bar (Numbers), which gets its name from the fifth word of the fifth verse of the book.
The Book of Numbers, ‘Be•mid•bar,’ means ‘in the wilderness.’
“And the Lord spoke to Moses in the wilderness of Sinai, in the Tent of Meeting, on the first day of the second month, in the second year after they came out from the land of Egypt, saying,”
If you are wondering why we consider the phrase ‘in the wilderness’ to be the fifth word in the first verse while in English it is the seventh, it is because of the conciseness of the Hebrew language where two and sometimes four English words equal one Hebrew word. It is partially because almost all prepositions in Hebrew are prefixes.