What does nr. mean?

[Russinating]

I've been sent a document which references numbers with nr., for example "20nr. months".

I can't Google it because it's so short, so what does it mean?

 

[Russinating]

It's to do with surveying so that makes the most sense, however I think "20 months" is pretty self explanatory without pointing out that the 20 is a number? What else could it be?