The free electrons in a circuit come from the outermost shell of atoms present in the conducting material.
The conducting wire is made up of atoms surrounded by electrons. The electrons cannot be created nor be destroyed. The electrons present at the outermost levels of the atom are loosely bond which allows them to move freely. The metal atoms become positively charged when these free electrons leave atoms. These freely moving electrons form a sea of electrons. In an open loop, this sea of electrons moves randomly.
But when the potential difference is applied across the wire, these electrons move in a closed-loop. The sea of electrons always moves from the negative terminal to the positive terminal such that the number of electrons in a small part of the wire remains the same at a given time.
It is similar to water (electrons) flowing in a closed loop of pipes (conducting wire) with the help of a water pump (battery). The amount of water (electrons) within the system will remain constant.
When the voltage difference is created, no electron is created or destroyed. The sea of electrons just moves in a closed loop. This movement of electrons in a close loop causes electricity to flow and even when the circuit is disconnected or switched off, the number of electrons remains constant in the circuit. They just stop moving in the closed-loop. So, the electrons are part of the existing circuit that moves through the circuit when a potential difference is created using a battery. The quantity of electrons remains the same as they cannot be created or destroyed.
So if you are wondering where do electrons in a circuit come from or how are electrons created?
The answer is electrons can neither be created nor destroyed. The freely flowing electrons are the part of the circuit and their number remains constant. They just move from positive to the negative terminal when the voltage difference is applied otherwise they move in a random direction. Therefore, electrons come from the material of the circuit.