Where does the formula I = -e/T comes from?

Yeah, where does it comes from?

 

[Orodruin]

You will have to be more specific and provide context.

 

[z_Positron]

I'm going to assume your talking about electric current?

In that case, electric current is defined to be the rate of charge passing a particular point.

Charge is given the symbol Q, time t. So if the rate is constant, the current I = Q/t.

In terms of your equation, -e is the charge of one electron, so your current would represent the rate at which a single electron (or something with that amount of charge) passes a particular point.