The measurement 0.090 L has three significant figures.
To break this down, the leading zeros before the first non-zero digit (which is 9 in this case) are not considered significant. They simply serve as placeholders to indicate the position of the decimal point. Therefore, the significant figures in this measurement are the digits 9 and 0 (the zero following the 9 is significant because it comes after a non-zero digit and indicates precision). Thus, the total count of significant figures is three: 9, 0, and the trailing zero (0).