The molecules containing covalent bonds between two atoms of the same kind are B2H6, N2O4, H2S2O3, and H2S2O8. Let's analyze each compound to understand why.

1. B2H6 (Diborane):
- It contains two boron atoms bonded together by a covalent bond.
- The formula B2H6 suggests that there are two boron atoms and six hydrogen atoms in the molecule.
- The boron atoms share a pair of electrons between them, forming a covalent bond.

2. N2O4 (Dinitrogen tetroxide):
- It consists of two nitrogen atoms bonded together by a covalent bond.
- The formula N2O4 indicates that there are two nitrogen atoms and four oxygen atoms in the molecule.
- The two nitrogen atoms share a pair of electrons, forming a covalent bond.

3. H2S2O3 (Thiosulfuric acid):
- It contains two sulfur atoms bonded together by a covalent bond.
- The formula H2S2O3 indicates that there are two hydrogen atoms, two sulfur atoms, and three oxygen atoms in the molecule.
- The two sulfur atoms share a pair of electrons, forming a covalent bond.

4. H2S2O8 (Peroxodisulfuric acid):
- It consists of two sulfur atoms bonded together by a covalent bond.
- The formula H2S2O8 suggests that there are two hydrogen atoms, two sulfur atoms, and eight oxygen atoms in the molecule.
- The two sulfur atoms share a pair of electrons, forming a covalent bond.

The remaining compounds, B3N3H6 and N2O, do not contain covalent bonds between two atoms of the same kind.

- B3N3H6 (Boron nitride hydride):
- It contains both boron and nitrogen atoms, but there is no covalent bond between two atoms of the same kind.

- N2O (Nitrous oxide):
- It consists of nitrogen and oxygen atoms, but there is no covalent bond between two atoms of the same kind.

Therefore, out of the given compounds, a total of four molecules contain a covalent bond between two atoms of the same kind.