Ask an autistic with OCD...
That was a cheap response, I apologize (it's insincere, but you deserve an apology.)
So, you can do it a couple ways. You could actually count the sand on the planet. As it's not infinite, and then count stars, until you hit 1 more... Or, you could do a bunch of really goofy math, crazy math involving a shit ton of boundaries, like "what is a pebble of sand?" It must be defined, as does "what is a star?" Count black holes? They're just super-dense stars, they've imploded and are actually brilliant, but the light can't escape the gravity (remember the "does light have mass thread? That's one way to prove light has mass... Only objects with mass can be affected by gravity.) How small can it be, before it's not a star? Does it have to be actively ignited? Can we count supernovas? And then, you figure out the size, approximately, of the universe, based on an expansion at the speed of light for 13.1 billion years, calculate galaxy density in the visible distance, and extrapolate from their, giving a margin of error of 50%+/-. With that margin of error in tact, and given math to approximate the "number of pebbles of sand" and then calculate the approximate number of stars. If the number show, unequivocally (i.e. double the margin of error, not mathematically, but numerically, and still have a difference greater on one side than the other.)
Basically, it's "theoretical," but, it's a pretty fuckin' solid theory.