Is the amount of substance of acid and base always equal at the equivalence point? Does the amount of substance of acid always have to equal the amount of substance of base at the equivalence point? I know it "works" for monoprotic acids and monobasic bases, but what if we had a diprotic acid and monobasic base?
We speak in this case of multiple equivalence points. 

As a concrete example, if we are titrating $\ce{H2SO4}$ with $\ce{NaOH}$, then the first equivalence point occurs when all the $\ce{H2SO4}$ has reacted to form $\ce{HSO4-}$, and the second equivalence point when all the $\ce{HSO4-}$ has further reacted to form $\ce{SO4^2-}$. If you started with 1 mole of acid in solution, then adding 1 mole of base gets you to the first equivalence point, and another mole (2 moles total) gets you to the second equivalence point. In the same vein, one might consider triprotic acids, which have three equivalence points, and so on. 

Incidentally, an interesting case to think about might be that of a titration between a diprotic acid and a dibasic base.