[mikegi]

Let's say I create a linearly polarized one terahertz electromagnetic wave (an ultrashort pulse, like those from photoconducting antennas). Is it a single photon or a collection of photons?

[Physicist]

Let's say I create a linearly polarized one terahertz electromagnetic wave (an ultrashort pulse, like those from photoconducting antennas). Is it a single photon or a collection of photons?

That depends how much energy you're radiating. Each photon has an energy equal to the frequency times Planck's constant. If you know the frequency and know how much energy you've put into the pulse, you can calculate the number of photons in the pulse. One is a perfectly good number; if you want a single photon per pulse, you just have to turn the intensity down to the correct level.