The calculation $N/V = p/k_\mathrm{B}T$ for the stated conditions can be done entirely with constants from scipy.constants:

In [x]: from scipy import constants
In [x]: constants.atm / constants.k / constants.zero_Celsius
Out[x]: 2.686780111798444e+25

This is the Loschmidt constant which is defined by the 2010 CODATA standards and included in scipy.constants (see the documentation for details):

In [x]: from scipy import constants
In [x]: constants.value('Loschmidt constant (273.15 K, 101.325 kPa)')
Out[x]: 2.686780111e+25