O=C(O)C1OC(Oc2cc(O)c3c(=O)cc(-c4ccc(O)cc4)oc3c2)C(O)C(O)C1O
A rotatable bond is defined as any single non-ring bond, attached to a non-terminal, non-hydrogen atom. Amide C-N bonds are not counted because of their high barrier to rotation.  How many rotatable bonds does the molecule have?