What speakers mean by it
In current English a number is a quantity, a mathematical object, or the written mark that stands for one. The same noun also names a place in a sequence, a telephone combination, an issue of a magazine, a song in a program, and, in grammar, the contrast between singular and plural. Informal speech can call a person, an outfit, or a practiced routine a number. The spread looks random until the history is set out. Most of these uses grew from a borrowed noun that already carried several jobs in Latin and French, then kept picking up new jobs as listing, printing, and telephones arrived.
Borrowed in Middle English
The origin of the word number is a Romance loan, not a native coinage. Around 1300, Middle English texts show nombre, noumbre, numbre, and similar spellings. Those forms follow Anglo-Norman noumbre and Old French nombre. Behind the French noun stands Latin numerus, a masculine of the second declension used for a count, a quantity, a collection of people or things, a rank, and, in more technical writing, rhythm, meter, and verse.
The Oxford English Dictionary classifies the English noun as a borrowing from French, with earliest evidence near 1300, including a citation in All Saints. Other etymological dictionaries allow that Latin may also have fed English directly, beside the French route. Either account still puts the immediate source in Romance. The first English sense is a sum or aggregate: how many items sit together, not yet the written digit.
Old English and early Middle English already had counting vocabulary. A native word related to tell and tale, written in Middle English as ȝetæl, covered quantity; rime was another competitor. The French form displaced them in the ordinary lexicon. Dutch nummer, German Nummer, Danish and Swedish nummer, West Frisian nûmer, and Icelandic númer look like close relatives, but they are later loans from Latin or Romance as well, not inherited Germanic twins of the English noun.
The verb entered in the same stretch of Middle English, from Old French nombrer, to count or reckon. By the late fourteenth century it could mean to assign a distinctive number; by the early fifteenth, to find out how many.
Latin shape, an inserted consonant, and a reconstructed root
Latin numerus is explained as a rhotacized outcome of reconstructed Proto-Italic *nomezos. A medial z became r, a regular Latin change. Handbooks then take that Italic form back to the Proto-Indo-European root *nem-, glossed as assign, allot, or take. The deeper step is a reconstruction, not a document trail, but it is the usual account in etymological dictionaries, including the American Heritage treatment of the English word and the Latin entry that cites Michiel de Vaan.
That root also yields Greek nemein, to deal out or distribute, and nomos, an allotted custom or order, along with nemesis. In Germanic it produced verbs of taking: Gothic niman, German nehmen, Latvian ņemt, and obsolete English nim. Numb and nimble sit on that Germanic line. English number is therefore only distantly related to numb; they are not one word that later split in sense. Old Irish nem, a gift, is another distant relative on the allotting side of the same root.
The conspicuous b in English and French is absent from the Latin stem. Old French developed nombre from numerus by inserting a b into the consonant cluster, a change familiar from other French developments. English kept that extra letter. The everyday English word and the more bookish relatives numeral, numerous, enumerate, numerator, and numerical all go back to the same Latin noun, but the learned set often came straight from Latin rather than through the spoken French form. Italian numero and French numéro are later cousins; English still uses No. from that line.
Latin already stretched numerus well beyond arithmetic. It could mean a military unit, a collection, a rank or position (to hold someone in a certain number), musical time, grammatical number, and a line of verse. The grammatical sense is treated as a semantic loan from Greek arithmos: Roman grammarians mapped the Greek idea onto their own noun. That learned meaning is in French by the thirteenth century and then in English. None of this requires a new etymology for each sense. The Latin word was already a cluster of counting, ranking, measuring, and grouping.
Later English branches
Once the noun was established, new uses followed technology and fashion more than a new source language.
Late in the fourteenth century, number could mean the written figure itself. From the late fifteenth century, and more clearly from the 1580s, numbers in the plural could mean measured verse or poetic meter, a continuation of the Latin poetic sense rather than a fresh invention. Number one for oneself is recorded from 1704; a mock-Italian numero uno is much later. The biblical book called Numbers, in English by about 1400, takes its name from the census that opens it. The Latin title is Numeri; the Greek is Arithmoi.
Magazine culture supplied a dated but still usable sense: a single issue as a number, from 1795. Telephone service produced the dialing combination around 1879, and wrong number by 1886. Theater programs listed acts by number; from 1885 a musical selection could itself be called a number. Colloquial speech by 1894 could treat a person or thing as a number. Childish number one and number two for bodily functions appear in the early twentieth century. None of these later senses needed a new donor language. They are extensions of labeling, listing, and counting.
Idioms keep the same skeleton. To have someone's number, attested in the mid-nineteenth century, is to have them figured out. One's number is up, from the early 1800s, points to a lottery ticket or a draft list. The American numbers game, an illegal lottery, is late nineteenth century. To do a number on someone, from around 1969, later meant manipulation, damage, or a practiced act, probably from the theatrical sense of a numbered routine.
The leftover idea of a share
The useful detail is not merely that English took a counting word from French. Latin numerus, and the reconstructed root behind it, sit on the idea of allotting a portion. The oldest English use is how much has been gathered or assigned. Far back, the same family of ideas also produced verbs meaning to take. English later hung telephone codes, magazine issues, and encore songs on the borrowed noun. The extra b, the French spelling, and native counting words that lost are the marks of how the word actually arrived.