Allow me to draw attention to my ignorance: I looked it up and I still don’t get the Avogadro constant. If it’s not too much trouble, could you explain?
Avogadro is one of the few people in the Universe who has a number all his own. People have house numbers, postal delivery route numbers, telephone numbers, and PINs. We are awash in numbers. Tsunamis of numbers inundate us on a daily basis. But we don't own them.
You can't copyright or patent a number. You may have favorite numbers, but they aren't yours. Numbers are a common heritage of mankind and all creatures with digits to count upon.
But Avogadro has his own number. Go to any library in the world, and ask about Avogadro's number, and you'll be told it's 6.02 x 1023. It's like knowing what the speed limit is, (186,000 miles per second).
Ahem, back to Avogadro.
Today, Avogadro's number is formally defined to be the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state.That's a fancy way to describe a lump of coal. And not even a very big lump of coal, either. Twelve grams wouldn't keep any of Scrooge's mice bookkeepers warm for long.
But it does give a "macroscopic" feel to the micro-microscopic world of atoms. An Avogadro's Number worth of silver atoms would weigh more, and be worth more, but it would be the same number of atoms as the lump of carbon mentioned above.
It's a really big number, but atoms are rilly, rilly small. In chemical calculations, being able to know exactly how many atoms are combining with exactly how many other atoms is profoundly informative and useful information, and that's what it's all about.