there are many small alveoli in our lungs instead of a few bigger ones because,  they would have a much greater surface area for the exchange of oxygen and carbon dioxide. (as greater the no.,larger the surface area.)
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hope ur doubt is cleared now.. :)
Alveolus are so small and uncountable because due to this in smaller size, exchange of gas can take place more easily. So Greater the no. of alveolus more easy to exchange gases.
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