A meter scale is graduated at every millimeter.

It is stated that the meter scale is graduated at every millimeter. Now, we know $1 \mathrm{~m}=1000 \mathrm{~mm}$. So minimum measurement can be of 1 digit value(say, $3 \mathrm{~mm}, 5 \mathrm{~mm})$, then there can be up to two digits and three digits(say, $76 \mathrm{~mm}, 305 \mathrm{~mm}$ ) and the maximum value can be $1000 \mathrm{~mm}$. So, there can be 1 , 2,3 or 4 significant digits.