Use SciPy's constants and conversion factors to calculate the number density, $N/V$, of ideal gas molecules at standard temperature and pressure ($T = 0\;\mathrm{^\circ{C}}$, $p = 1\;\mathrm{atm}$). The ideal gas law is $pV = Nk_\mathrm{B}T$.
The calculation $N/V = p/k_\mathrm{B}T$ for the stated conditions can be done entirely with constants from scipy.constants:
In [x]: scipy.constants.atm / scipy.constants.k / scipy.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