Final answer:
The number indicating eggs from hens reared in the open air corresponds to a 'free-range' classification, often designated by the number 1 in European egg labeling systems.
Step-by-step explanation:
The number given to eggs from hens reared in the open air is not explicitly provided in the materials provided. However, this type of question would typically relate to egg labeling practices that designate the conditions under which the hens are raised. For example, in the European Union, eggs from hens that have outdoor access are often labeled with a code that begins with the number 1, indicating 'free-range'. The question seems to be seeking such a classification code rather than a numerical value related to the eggs themselves, such as quantity, size, or nutritional content.