The number of electrons shared between the two Boron atoms directly in the formation of bonds in diborane molecule

Number of electrons shared between two boron atoms in diborane is zero.

In diborane each B atom undergoes sp³  hybridisation ,it contains two B-H-B bridges . Each bridge bond is formed by overlapping of one "B"sp³  hybrid orbital with one electron, hydrogen s-orbital with one electron and another boron sp³ hybrid orbital without electron .That is the bridge head bond is a electron deficient three centre two electron bond.

The number of atoms involved in bridged bonds in one diborane molecule is four.