The density of an object with mass of 50.0g and volume of 2.5 cm^3 should be reported with 2 significant figures.
Density is mass/volume. The rule for multiplication and division is that the answer is rounded to the same number of significant figures as the data with the least number of sig figs. The mass is reported to three sig figs. Zeros at the end of a number are significant when there's a decimal point. The volume is reported to 2 sig figs, as all non-zero digits are significant. The volume measurement has the least number of sig figs, 2, so the density should also be reported to 2 sig figs.
The multiplication and division rule doesn't apply to addition and subtraction. When adding or subtracting you round the answer to the same number of decimal places to the right of the decimal point as the datum with the least number of places to the right of the decimal point.