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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

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 21.210^27 and 62.410^27 eV).

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

This makes between 10.610^27 and 31.210^27 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

After the correct comments, you are asking about the number of photons. Now it is only possible to calculate if you use an average wavelength (frequency). I chose visible light wavelength. This calculation can be done with a different wavelength too.

There is a misunderstanding, and that is what the lightning is made up of. It is made up of electrons. Some of the energy will end up as photons, but not all. Assuming that all energy would be dissipated as photons, we could calculate a number. Of course this is not correct, since you are asking how many photons are in the bolt.

This calculation could only be theoretical, and after the comments it does not make sense to talk about photons in the bolt since it is made up of electrons. I am showing here a calculation, but again, this is only theoretical for the question, in reality the bolt is made up of electrons, not photons.

Assume visible photons, there is 2 eV in a photon.

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 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

After the correct comments, you are asking about the number of photons. Now it is only possible to calculate if you use an average wavelength (frequency). I chose visible light wavelength. This calculation can be done with a different wavelength too.

There is a misunderstanding, and that is what the lightning is made up of. It is made up of electrons. Some of the energy will end up as photons, but not all. Assuming that all energy would be dissipated as photons, we could calculate a number. Of course this is not correct, since you are asking how many photons are in the bolt.

But let's disregard that.

Assume visible photons, there is 2 eV in a photon.

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.

After the correct comments, you are asking about the number of photons. Now it is only possible to calculate if you use an average wavelength (frequency). I chose visible light wavelength. This calculation can be done with a different wavelength too.

There is a misunderstanding, and that is what the lightning is made up of. It is made up of electrons. Some of the energy will end up as photons, but not all. Assuming that all energy would be dissipated as photons, we could calculate a number. Of course this is not correct, since you are asking how many photons are in the bolt.

This calculation could only be theoretical, and after the comments it does not make sense to talk about photons in the bolt since it is made up of electrons. I am showing here a calculation, but again, this is only theoretical for the question, in reality the bolt is made up of electrons, not photons.

Assume visible photons, there is 2 eV in a photon.

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.