Why is number abbreviated as no When there is no o in number?

Answer 1

Answer: Abbreviation for Number

It's just what people chose for the abbreviation for that word, like lb for the word pound. There is no l or b in pound.

But the logical answer is:

In fact, no. (once also written with a superscript o) is an abbreviation of Latin numero, the ablative of numerus 'number'; the ablative case is used for the meaning 'in number', which is how the abbreviation was first used.

Your question now will no doubt be "OK, then, why is 'no.' the abbreviation for 'numero'? Why not 'nu.'?" The answer to this is that abbreviations don't necessarily have the initial part of a word, especially if that's not the important part. So "Mister" is Mr., not Mi., and "verb" is vb. Knowing that numero has an -o ending is important, and that's why we have no.

The abbreviation no. is first found in English in the seventeenth century.