According to [Could We Harness Lightning as an Energy Source?][1]:

> An average bolt of lightning, striking from cloud to ground, contains roughly one billion ($1,000,000,000$) joules of energy.

According to [Visible light][2]:

> Red photons of light carry about $1.8$ electron volts (eV) of energy, while each blue photon transmits about $3.1$ eV.

So let's take an average photon energy of $2.5 \text{ eV}$.  
Then we can calculate the number of photons in the average lightning.

$$ N = \frac{10^9 \text{ Joule}}{2.5 \text{ eV}}
     = \frac{10^9 \text{ Joule}}{2.5 \cdot 1.6 \cdot 10^{-19} \text{ Joule}}
     = 2.5 \cdot 10^{27}$$

 [1]: https://www.realclearscience.com/blog/2012/05/could-we-harness-lightning-as-an-energy-source.html
 [2]: https://scied.ucar.edu/visible-light