A metre scale is graduated at every millimetre. How many...

The least count of a metre scale graduated at every millimetre is $1$ mm.

The measurements made with this scale can range from $1$ mm to $1000$ mm.

If the measured length is between $1$ mm and $9$ mm, the number of significant digits is $1$.

If the measured length is between $10$ mm and $99$ mm, the number of significant digits is $2$.

If the measured length is between $100$ mm and $999$ mm, the number of significant digits is $3$.

If the measured length is exactly $1000$ mm (or $1.000$ m), the number of significant digits is $4$.

Thus, the number of significant digits in a length measurement made with this scale can be $1, 2, 3$ or $4$.