The respective number of significant figures for the numbers 23.023, 0.0003 and 2.1 x 10^-3 are

The reliable digit plus the first uncertain digit is known as significant figures. For the number 23.023, all the non-zero digits are significant, hence 5. For the number 0.0003, number is less than 1, the zero(s) on the right of decimal point but to the left of the first non-zero digit are not significant, hence 1. For the number 2.1 x 10^-3, significant figures are 2.