Avogadro's Principle
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Avogadro's Principle is a fundamental assumption in statistical mechanics that all ideal gases at equal volume, temperature and pressure have the same number of molecules.
Avogadro's principle is a principle rather than a law (a summary of experience) because it depends on the validity of a model, in this case the existence of molecules. Despite there now being no doubt about the existence of molecules, it is still a model-based principle rather than a law.