When we mix NaOH and H2SO4 we get the equation

Question

OH-(aq) + H+(aq) --> H2O(l)
and if we mix H2CO3 and NaOH we get the equation
2OH-(aq) + H2CO3 --> 2H2O(l) + CO3-2(aq)

Why aren't the two equations the same? ie only one mol of NaOH reacts in the first equation but two mol of NaOH in the second - Why?

The difference is that the second acid is diprotic, so it takes two OH ions to neutralize all the hydrogen ions in the molecule.

According to my textbook H2SO4 is diprotic too.

That's correct. H2SO4 is diprotic; H2CO3 is diprotic. BUT H2SO4 is a strong acid; H2CO3; is a weak acid. Your first net ionic equation involves only H^+ and OH^- because both NaOH and H2SO4 are 100% ionized (actually, H2SO4 has a relatively large k2 of 1.2E-2 but for most freshman work we consider H2SO4 100% ionized).In the second equation, H2CO3 appears because it is a weak acid