Avogadro made his hypothesis in 1811, following earlier proposals by Gay-Lussac that when gases reacted together they did so in simple ratios.  The first estimate of Avogadro's number was in 1865, following work of Loschmidt, and since then there have been a wide variety of methods used to improve the accuracy which we can give to this number, and experiments were made by some of the most prominent people in physical science. For  their time the experiments showed great originality and were made with extraordinary care.

Avogadro's number,  6.0221367x10(23) is a great example of nearly 200 years developement  of physical science.