${\mathrm{H}}_2{\mathrm{CO}}_3$ionises in two stages as represented below

${\mathrm{H}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{HCO}}_3^{-};{\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CO}}_3^{-2}$

the no.of conjugate acid-base pairs in the above reaction are

${\mathrm{H}}_2{\mathrm{CO}}_3+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {{\mathrm{HCO}}_3}^{+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$

Here,${\mathrm{H}}_2{\mathrm{CO}}_3,{\mathrm{H}}_3{\mathrm{O}}^{+}$are proton donors and ${\mathrm{H}}_2\mathrm{O} \mathrm{and} {\mathrm{HCO}}_3^{-}$ are proton acceptors. In 2 nd reaction, ${\mathrm{HCO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CO}}_3^{-2}$

${\mathrm{HCO}}_3^{-}, {\mathrm{H}}_3{\mathrm{O}}^{+}$ are proton donors and ${\mathrm{H}}_2\mathrm{O}, {\mathrm{CO}}_3^{-2}$are proton acceptors. There are 3 conjugate acid-base pairs.