Let's guess that a crumb of toast weighs a hundredth of a gram, that is 0.00001 kg. Let's also assume that, being organic, toast is mostly made of carbon, oxygen and hydrogen, with an average atomic weight of about 8. (C=12, O=16, H=2, but there are more hydrogen atoms than carbon or oxygen). So let's simplify our toast by pretending that it's made up of atoms with atomic weight 8. By definition, one mol of anything weight as much in kg as the atomic weight of the things, so a mol of toast atoms weighs 8 kg; and one mol is a number of things equal to Avogadro's constant, which is 6 x 10^23, or 600,000,000,000,000,000,000,000. But we don't want 8 kg of toast, we only want 0.00001 kg, so we divide that number by 8 and multiply by 0.00001 and we get a rough answer: 750,000,000,000,000,000 atoms, or 750 quadrillion.