Which is the correct sentence?

"The number of cases has increased" is correct. Perhaps you copied one of those sentences incorrectly from your source, because as you have posted that phrase in both sentences. Are you asking about "Shouldn't it be?" The choice between them would be stylistic, either sentence could be used, they are both correct. I would prefer the second sentence, but the first is not incorrect.

Thanks, Setanta

I am asking whether it should be or it shouldn't be.

You have concluded correctly what I wanted to ask.