A carbon atom typically possesses six electrons – two in its inner shell and four in its outer shell. This number varies due to a number of circumstances, but a stand-alone atom with no charge contains six electrons.
The number of electrons in a carbon atom's shell can vary due to possessing a charge and becoming an ion or in combining with other atoms. Interaction with particles and other atoms can temporarily knock an electron out of its orbit or add an additional one, giving the atom an electrical charge and making it an ion. Carbon also has four open spaces on its outer electron shell, allowing it to combine easily with other atoms and share extra electrons with them.