There wasn't 7 major parties. Five maximum, even two could be argued. But '33 Germany is a weak argument against multiparty systems. Interwar Germany was not a well functioning democracy at all. They had armed street fights and deep political chaos going on for over two decades at that point. Hitler didn't have the majority and formed a coalition government. Only because Hindenburg agreed to dissolve the Reichstag could the nazis take power fully.

So the number of parties did actually block Hitler, and Presidential powers to subvert democracy was the problem. In modern multi party democracies an inability to form a government will result in a new election, not installing a dictator.