How many significant figures (sig figs) does 0.050400 have? also can you explain why?

Robbie's wrong. There are 5 significant figures. No leading zeros are ever significant (start counting at the 5 in this case). Also, it's not about "precision," it's about how accurate the measurements are. This specific measurement implies that the observer knows with certainty that something is .05040, and has estimated the final 0.