Are there more atoms in one grain of sand than there are stars in the known universe?

No. Sand grains vary quite a but in size, but for our purposes let us consider a spherical grain of sand composed of quartz that is 1 millimeter in diameter. Such a grain would have a mass of about 0.0014 grams and given the formula of SiO2, with a molar mass of 60 grams per mole, would contain about 4.2*10^19 (42 quintillion) atoms. It is hard to estimate the number of stars in our universe and there do not appear to be any reliable sources. There are about 100 billion galaxies in the known universe and own galaxy contains an estimated 200 billion stars, but it is significantly larger than the average galaxy. If we figure the average galaxy contains about 20 billion stars then we get an estimate of 2.0*10^21 (2 sextillion) stars in the known universe universe, about 50 times the number of atoms in our sand grain.