The same way you would do it in English? It's obvious from the context in both languages. "He boils an egg." means that there is one egg exactly, otherwise it would be "(a few) eggs" so if you for some peculiar reason wanted to stress that the quantity of eggs boiled is one (1) then you have to say the same thing you would do in English; "exactly/precisely/only one egg."
Actually, I suspect I might be wrong. ONE egg might be "Uno uovo". Not sure if "uno" becomes "un" before a vowel. The indefinite article "un" becomes "uno" when it's followed by a vowel indeed, but I'm not sure if it works the other way round as well. Can anyone help us?
EDIT: geez.. sorry about that. The indefinite article "un" does NOT become "uno" when it's followed by a vowel. It stays as "un" before the vowel. It becomes "uno" when it's followed by "z", "s+consonant" and "ps". Please ignore what I said above. Still, I'm not sure if "uno" which means ONE changes to "un" when it's followed by a vowel.