Oh wow, that’s a hard one to answer! It depends a bit on which particles you’re thinking about. If you think about cells as particles, then an adult contains about 30 trillion human cells, as well as 40 trillion bacterial passengers! If you want to think about particles much smaller than that then it quickly gets impossibly tricky. You could think of chemical molecules as particles, but it would probably be impossible to guess how many of them there are with any kind of accuracy! For example, an adult might contain about 55 litres of water. Each litre contains about 3×10^25 (that’s 3 followed by 25 zeroes) water molecules, so in the whole human body that’s 165×10^25 water molecules – but there are way more chemicals in your body than just water!
And, if we want to just get a very silly number, we could think of individual atoms as particles – there are three in each water molecule, so there are roughly 4,950,000,000,000,000,000,000,000,000 atoms in just the water in our bodies! We could go on (there are 28 protons, neutrons and electrons in each water molecule for example), but given we’ve already reached a number that’s in the octillions maybe we should stop..!
So how many particles are there in your body? Short answer: a lot!!