||There is no way to know exactly, because of several variables, but one can make a reasonable estimate because the field of chemistry has some standards which make this pretty easy (molecular weights and Avogadro's number).|
First, LSD sold on the street can refer to several different distinct chemical salts of LSD. We'll ignore the issue of breakdown products and other non-LSD chemicals that might be in street acid, you can read a little about that in the article "LSD Analysis: what do we know about street acid?".
The simplest form of LSD is called the "free base" which means that it is just the LSD molecule itself with no stabilizing salt. The LSD sold on blotter or in other forms on the street is often 'attached' to a salt. One of the most common forms is LSD tartrate, or an LSD molecule with tartaric acid acting to make it a salt.
The things one needs to know to calculate the number of molecules in a mass of some chemical are: the amount of the chemical in question, its molecular weight, and Avogadro's number (a special pre-calculated number). Each unit of molecular weight, or "mole" (abbreviated mol) of a chemical is equivalent to the mass of that chemical that contains about 6.022 x 10^23 molecules. (602,200,000,000,000,000,000,000)
An important consideration when working with these numbers is that the numbers are incredibly large. Because of this, rounding errors will generally not affect the final result by much. We're hoping to end with a number that is within an order of magnitude of the correct number (something with the same number of zeroes after it).
We'll start by calculating how many molecules there are per microgram (ug).
The molecular weight for LSD tartrate is 398.485 grams per mol. If you can't just look this up, it is a little tricky to figure out. Generally, one just takes the molecular weight of the freebase and adds the molecular weight of the salt component, but it is safer if you can find a reliable reference that just provides the final number since there are some salts that do not bind 1-to-1 with the freebase chemical and this is true for LSD tartrate.
We restate the molecular weight more mathematically as 398.485 g/mol. We want the answer in the form of molecules per mass (not mass per molecule as it is now), so we turn the fraction on its head. That is 1/398.485 mol/g, which is .002509504749 mol/g. Since we want to end up with micrograms as our unit, we divide by 1,000,000 (one million micrograms per gram) or, equivalently, multiply by 10^-6. So that is .002509504749 x 10^-6 mol/ug (mols per microgram).
We'll round this and restate this as: 2.51 x 10^-9 mol per microgram. Then we take Avogadro's Number (6.02214199 x 10^23), which is the number of molecules per mol, and multiply the two together: 2.51 x 10^-9 times 6.02214199 x 10^23 to get around 1.511 x 10^15.
For symmetry and human memory's sake, we can simplify this to 1.5x10^15 molecules LSD tartrate per microgram. That is 1,500,000,000,000,000 or 1.5 quadrillion molecules of LSD tartrate per microgram.
To get how many molecules there are in 100ug, one simply multiplies by 100 to get 1.5x10^17 or 150 quadrillion molecules of LSD.
Summary: there are lots and lots and lots of molecules that make up even the tiniest quantity of substances.