1. I suggest you try it.

Question

2. I demand you try it.

(Are both all grammatical?)

3. How many floors is the second floor from ground?

(Is it grammatical? What will be the answer to this question?)

Answer 1

1 and 2 are grammatical. There's a big difference, though, between the meanings of these sentences.

3. Is also grammatical. The second floor is usually the second floor above ground. However, I enter my apartment from the ground level, but because the ground slopes, the back of my apartment is actually one floor above ground level. Some buildings may have a ground floor, making the second floor two floors above ground level.