How to type № on iPad?

I don't think that you can type that symbol on the iPad's keyboard - you can do N° on the keyboard (° is on the 0 key) or just No. . A slow alternative if you want to use it is to copy and paste that symbol from here or a website into a note on your iPad and you can then copy and paste it from there whenever you want to use it - but obviously that will not be quick. Otherwise you may be able to get it and other symbols via a third-party app such as Unicode Maps, but again it would be a case of copying and pasting from there in order to then use it where you want it to appear.

I don't believe that there is any keyboard shortcut to this Unicode character. If you are using the Spanish keyboard you can just type N° (N followed by the ° zero key option) or you could get one of the Unicode Apps like Uniconsole which allow selection and copy of the Unicode characters like №.