Solution:

A coin ends as tails if and only if it has $3$ flips that are tails, which happens with probability $\frac 18.$ Thus, the probability of any coin being heads is $\frac 78 .$

As the probability that a given coin flip is $\frac 78,$ and there are $64$ coin flips in total, the expected number of coins that are now heads is: $$64 \cdot \dfrac 78 = 56.$$

Thus, the correct answer is D .