How many three-centre two-electron (3c-2e) bonds are present in one molecule of diborane, $\mathrm{B}{}_2\mathrm{H}{}_6$?

Diborane has two bridging hydrogens, each forming one B-H-B banana bond (3c-2e). The four terminal B-H bonds are ordinary 2c-2e bonds. So there are 2 three-centre bonds.