What is the difference, if any, between "I've got a brother / a dog ...." and "I've got one brother / one dog ...."?
My students often use "a / one" interchangeably.
As you know, the problem is that in many languages, including Italian, the indefinite article 'a' and 'one' share the same word.
In English, we use 'one' when the number has a particular relevance.
'He has a brother called Hector, hasn't he?'
'He has two sisters but only one brother.'
'You owe me a thousand pounds.'
'Is it just one thousand pounds you owe me?'