Now after the question has been clarified, that is, you are asking for the number of photons emitted by the lightning, and seeing other answers likewise as mine before, I will try to edit the answer (citing references).

Assume visible photons (like in other answers too, I assumed the range average to be 2 eV), there is 2 eV in a photon (your question does not state visible or non-visible, but it would be very important to distinguish).

Now as Emilio Pisanty's answer seems to be the most correct one, stating that:

1. there is no consensus on what portion of the lightning's energy is converted into photons and other forms of energy dissipation

2. it is very important to understand that you are not specifying whether you are talking about visible or non-visible photons 

A lightning has between 5 and 10 billion Joules (between 21.2 and 62.4*e^27 eV).

https://en.wikipedia.org/wiki/Harvesting_lightning_energy

This makes between 10.6 and 31.2*e^27 photons.

Now I have a very interesting additional reference for the spectrum of the lightning's emitted photons:

http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964ApJ...139..994W&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf