The greatest number of valence electrons that can be shared between two main group atoms in a covalent bond is eight.

Here's why:

* Valence Electrons: These are the electrons in the outermost shell of an atom, which are involved in chemical bonding.

* Octet Rule: Main group elements tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons (like a noble gas).

* Covalent Bonding: Atoms share electrons to achieve this stable configuration.

Example:

Consider the molecule of oxygen (O2). Each oxygen atom has six valence electrons. To achieve an octet, each oxygen atom shares two of its electrons with the other oxygen atom. This results in a double bond with a total of four shared electrons (two from each atom).

While it's possible for atoms to share more than eight electrons in some cases (especially with transition metals), this is not common for main group elements.