How many significant digits are there in 17.0

The answer is three significant figures. The 1 and the 7 are both significant, because they are non-zero quantities.

This is where significant figures gets a little more complicated, because if a zero is used as a placeholder (i. e. 0.00027 cm) then it is insignificant.

But in the case above, the zero isn't being used as a placeholder, and thus, is significant.