When you see "No." as an abbreviation for number, do you wonder why? Why not "Nu." or just plain "N"? It's from the Middle English days, when the word was nombre. The abbreviation has stayed but Modern English turned the word into number. Always include the period, though!
The correct abbreviation for number is "No." and it is used only in front of an actual number, e.g.:
No. 1 / Paragraph No. 2.