Lets start with something smaller than the Universe.
The largest size that geologists (people who study the earth and rocks) take for a grain of sand is a ball with diameter 2mm, so its radius would be 1mm. The volume of this grain of sand is calculated using the formula Volume = (4/3)*pi*(radius*radius*radius) = (4/3)*(3.14)*(radius*radius*radius) = 4.2 (approximately). So.. our grain of sand has a volume of 4.2mm cubed.
The Earth has a volume of approximately 1 x 10^12 km cubed (you can read that as 1 by 1o to the power of 12, which is 1 with 12 zeros after it which is equal to 1 x 10^30 mm cubed (thats 1 with 30 zeros after it). If we let all the grains of sand pack into the earth perfectly leaving no little gaps between them, then that means we could fit (1×10^30 / 4.2) of our sand grains in to it, which is about 2.4 x 10^29.. so lots. Of course, round balls dont pack perfectly as there is always space between the curves, so it would be a bit less than this, but I have already rounded so many of the numbers that we are probably close enough to be happy for now.
As for the universe… well it may well be infinite… but the observable part around earth apparently has a radius of about 46 billion light years, giving it a volume of pi*46*46 = 47,720 billion light years cubed. If i start putting grains of sand or even entire Earths into that, im going to run out of space for all the numbers… so lets just leave it as a huge number 🙂
I think Andrew has given you a great answer. If you like these sort of questions you should try and do some Fermi questions – these are ‘figure things out’ style questions that you should be able to do with a pen and paper and maybe a calculator (try not to google things – test your ability to guess). Can you work out
1. How many times your heart will beat in your life time?
2. At what speed does your hair grow?
3. How many leaves will fall in Dublin this autumn?
4. How many piano tuners live in Dublin?? (this one is a classic),