If you say anything beyond 1... people will enquire.
My personal decision:
I lost a little girl at 6 months, and over 10 pregnancies in the first 4 months, most of whom I don't even know the sex of. I don't tell people I have 12+ children, I tell them I have my kiddo UNLESS I want to be talking about miscarriages with people. Even then, I am VERY careful to let it be known that I miscarried. Having both a child I held in my arms, and those I did not... the pain is entirely different for *me* at least.
Working in hospitals I've seen far too many people waving and kissing goodbye to their children (infant to toddlers to teens), to allow any blurring of lines. I would fear far far too much that if I said anything along the lines of "in heaven" or "would be x years old" it would be to a mum who'd buried her toddler. I love my son so much more than the children I've miscarried... If he died, I wouldn't be long after... I couldn't see living in a world without him. That that particular line in the sand between born and unborn may as well be a line in stone. I do not cross it.
I also lived over 1/3 of my life in Japan. THERE they have cemetaries for unborn children, which I miss here. There is an understanding of how painful it is to lose a child you've never raised, and all of the thoughts of "what would they be like?" being huge in a mother's life. So here... where we don't have that, culturally speaking, I do it with words: I have 1 son, and I've miscarried many.
Please note all of the "I" statements. These are just my personal decisions, based on my own experiences... I am in no way saying what anyone else should do. IMHO what a person *should* do, is what makes them feel right/happy. This is what feels right to me. To another, the answer may be entirely different.