It is hard to estimate how many people are being held in private domestic prisons: basements, back rooms, fenced-in tents and huts in back gardens. The main difficulty is that the initial class, missing persons, is very large. Many thousands go missing every year, presumable to run away from disliked parents. Mostly they are teenagers just running away from home. Most of them return, or at least communicate with home later on, but some do not, or not for many years. This is one factor which makes it difficult to find missing persons. The other difficulty, bluntly, is that it is relatively easy to hide prisoners in domestic settings, particularly in places where there are weak community ties. Ties have to be strong enough for neighbours to know exactly who has children or relatives staying with them, and something about the organisation of the household. That is an unusual level of knowledge to find in anonymous suburbs. Further, most wealthy societies have spare rooms, and even small families can buy enough food and clothing for two families without raising any suspicion.
Of course, these terrible cases seem to be very rare, but given the above circumstances it could be that it is only their detection which is very rare. Perhaps the best estimate would be to guess what fraction of children who are still missing after a year have been murdered (probably lower than 1%) and then what percentage are captive (an even lower percentage, I would imagine, but on no very solid grounds other than perpetrator's fear of being found out might be greater than the benefits of keeping slaves).
By the way, the press often ask: what is it about Cleveland, Austria, Gloucester etc which has allowed these crimes to happen? Nothing. Most people live in suburbs, don't know everything about their neighbours, don't think anything awful is happening in their street, and are usually right.