Thanks to observations by telescopes such as Hubble, we can estimate that there are over one hundred billion galaxies in our universe – and they’re just the ones we could see. So, by knowing the proportion of the sky that the grain of sand is covering, we can estimate the number of galaxies in that area.
Galaxies are huge swirling masses of stars, dust, and dark matter, and each galaxy can contain anywhere from 10 million stars (for a dwarf galaxy) to a thousand billion stars (for a giant galaxy). So even if your grain of sand was hiding only dwarf galaxies, it would still cover around a hundred billion stars!
Read more on this awe-inspiring fact at Physics.org