The expected number of heads in 500 tosses of an unbiased coin is:

There is a 50%, or 1/2, chance of getting heads on each flip. Each of the 500 trials is independent. Because there is a 1/2 chance of getting heads, you would expect half of them to land on heads. 500/2 = 250. You would theoretically expect  250 heads if you tossed an unbiased coin 500 times.