How many significant figures does the number 520 have? what is the difference between a systematic error and a random error?

I'd say there are two significant figures in the number 520, and those are 5 and 2, because trailing zero in a number without decimals is rarely considered to be significant.
Now, the difference between a systematic error and a random error is that a systematic error occurs over and over again, whereas a random error is random and may never be repeated.