${\left(S{i}_2{O}_5\right)}_n^{2n-}$ anion is obtained when:

No oxygen of a $Si{O}_4^{4-}$ tetrahedron is shared with another $Si{O}_4^{4-}$ tetrahedron

One oxygen of a $Si{O}_4^{4-}$ tetrahedron is shared with another $Si{O}_4^{4-}$ tetrahedron 

Two oxygen of a $Si{O}_4^{4-}$ tetrahedron are shares with another $Si{O}_4^{4-}$ tetrahedron

Three oxygen of a $Si{O}_4^{4-}$ tetrahedron are shared with another $Si{O}_4^{4-}$ tetrahedron