We are asked how many significant figures we have in an exact number.
The answer depends on the position of the decimal point (and of course if there is a decimal point)
1) If there IS a decimal point given with the value, the number of significant figures is that of the total number of digits different from zero (counting from the left. And after the first significant digit, you count all the digits that follow to the right.
For example: 0.000450680 has a total of 6 (six) significant figures these are:
450680 We don't count the zeros to the left which are "placing zeros"., but we do count all the zeros that appear after the first non zero digit as significant figures.
2) If there is NO decimal point in the exact number, like forexample:
135000 In this case there are just 3 (three) significant figures , and these are: