This is one of the most difficult questions. As always, the absolutely accurate answer is impossible as, even when the housing industry is broken and the existing dwellings resist (i.e. Spain today), there is always a building at works somewhere adding some units, or a ruin, or a demolition.
The housing censuses were considered to be precise; but today in many countries (Spain included) they are made with a representative sample, not the entire stock. They are made taking account of the cadastre, a fiscal data which is exhaustive, but that sometime does not show exactly what you are looking for (a housing building with a single owner that rents 100 flats can be fiscally registered as a single property, or a parking slot be registered as a housing property). Sometimes a dwelling is divided in many by its owner, without any registration, and the opposite case (grouping neighboring homes) is also possible.
Even in areas with recent buildings, where the number of dwellings is defined by plans, there can be fluctuations, upside or downside: a dentist can occupy what was to be a home, and there can be someone in fact living in what was to be an office… again, the difference between the normative world and the real facts of life…
As in the rest of the examples of this week, it is always possible to get a figure; what is needed is the knowledge on how this figure was produced, as to be able to relate that to other figures and give it an operational sense. A figure by itself is usually rather unrepresentative.