An atom has six electrons in its valence shell. how many single covalent bonds would you expect it to form in most circumstances?

The atom will form two covalent bonds under normal circumstances.

The principle behind the number of bonds formed under normal circumstances is simple. The atoms wish to attain a stable atomic configuration by completing their valence shell by having 8 electrons (under most cases) in it. The best way to do this for the given atom is to gain 2 electrons, rather than try to lose its 6 electrons because losing so many electrons will require more energy.