Explanation

In $23.023$ number of significant figures will be $5$ because all the zero’s between non zero digits are significant. In $0.\ 0003$, number of significant figures will be one because all the zero’s before and after decimal point are insignificant if the number is less then one. In $2.1 \times 10^{- 3}$ number of significant figure are to because power of 10 is not considered as significant figure