In Europe when the word " number " is abbreviated they write Nr. which makes good sense as both letters are in the word. But we would write No. as in No. 28 North street. Why the the letter "o" .
But F nombre is number in the quantitative sense. For No. 28 etc. they use num�ro, which they also abbreviate no, or rather usually n�. It prob comes from both the ablative of Clanad's Lat numerus and Italian numero as in numero uno. These and other Romance forms all come from that Lat numerus, but German goes in for 'linguistic purism' and abbreviates the German form Nummer as Nr.
For as long as it takes to answer the question properly, young carl, and perhaps even to get through to people who believe that exclamations marks are the only form of communication worth knowing about.