Enough for 3 to 4 million people.
So says the government, and most of them will be "affordable" homes, whatever that means. Given the average wage outside of London and the lenders' multiplier they will have to come in at ~£175k. I cannot see many developers wanting to get involved at those prices so "affordable" becomes "pricey".
We also have a penchant for living on the ground so most of these homes will be detached or semi-detached with a few mews and town-houses thrown in. Where will the land come from? Say an average plot was 150sq/m (10x15) the total 1 million home plot size would be 150sq/km, add in the roads, pavements, verges and gardens and we are looking to double that - an area of 17.4km x 17.4km. To put that into perspective it is a square from White City to Poplar, E to W, and from Tottenham to Dulwich, N to S.
If the population keeps rising we will have to start all over again in 2020 - 2025.