See subject. Or is this a nonsensical question?
I was trying to be clever in another thread and throwing terms about.
Thanks,
Leo
According tothis wiki article:
Approximately 6.42 x 10**61 (rounded).
(I have no idea how they came up with that number.)
Just brute force counting. I’m already up to 14.
15… 16… 17…
For some reason, at the bottom of this thread, there are two Google Ads for brain injury lawyers. 
But the first 12 have already gone out of existence. Start again, and count more quickly next time.
Dammit, now I lost my spot. Ugh.
1… 2… 3…
There’s only one and it just gets passed around. Just like fruitcake.
Except fruitcake has infinite mass.
I recall reading somewhere: a new hypothesis posits that there’s only one photon, but it’s able to travel back and forth in time
Not sure about about a new hypothesis, but the old one is from Feynman, who made the joke[?] about an electron.
That argument just about almost works for electrons, but not for photons. Photons aren’t associated with any conserved quantum numbers, and thus can be and are routinely created and destroyed.
As for how it’s determined, you can calculate the flux of photons due to a blackbody of a given temperature. Plug in the temperature of the CMB, and you’ve got a good approximation of our Universe (the CMB accounts for the lion’s share of photons). Divide by c, and you turn the flux into a number density. Multiply by the volume of the observable Universe, and you get the total number.
Another beautiful hypothesis slain by an ugly fact.
Thanks for the enlightenment about photons, you two. 