I understand the SI meaning of a Coulomb but how were the individual electrons counted in the measurement to know how many it would take to constitute a Coulomb?
Please check the following references at the bottom (from the earliest known usages of Coulomb in unabridged Oxford English Dictionary). Also, check the early definition of ampere which was also defined on the basis of silver nitrate electrolysis.
from the wt of Ag+ deposited, by Faraday's law, we have one to one correspondence:
Ag+ (solution) + electron ---> Ag (metal), thus from the weight of Ag deposited during electrolysis, you can count electrons involved in the process.
You can independently measure charge passed through a solution via time (t) and current (I). Q= Ixt
1881 Nature 29 Sept. 512/2 The name Coulomb to be given to the quantity of electricity defined by the condition that an ampère gives one coulomb per second.
1892 Lightning 3 Mar. (Spec. Suppl.) Gloss. Electrical Terms s.v. The Coulomb is the quantity of electricity, which, when passed through a solution of nitrate of silver, deposits ·001118 of a gramme of silver on the plate by which it leaves the liquid.
This was discovered in the experiments with electrolysis. The idea is that when electric current passes through a solution, positive ions accumulate on one side, and negative on another. We know the charge in terms of the number of electrons of one ion (+n if n electrons are missing, -n if there are n extra electrons). The number of ions that accumulated can be found when you know their atomic weight and the weight of the accumulated material. On the other hand, by measuring time and current passing through the solution we can find out how many Coulombs correspond to this charge.