Q:

How many degrees of freedom does the air have? 

Very simple question that I am overthinking... But how many degrees of freedom does the air have?  Assuming let's say the air is confined in a rigid box.  

A:

This answer is to address the comment by @El borito

Excuse me, why the degrees of freedom for diatomic molecules are 5 in a room temperature? 

The two rotational degrees of freedom arise from the fact that the dipole can rotate in two different planes. 
The three translational degrees of freedom are due to motion in the $x$, $y$ and $z$ directions (if working in cartesian coordinates)

[Remark: This image is the same as an answer given to this question which explains in more detail so should be consulted]

