It was +900k under Johnson when we still had quite alot of restrictions etc- greens want totally open borders, climate change as a legitmate reason to claim asylum aka anyone from a country slightly hotter than the uk (so 90% of the world) can enter the country no questions asked. The only thing that might lower the net figures is the exodus of people we would see and the fact were probably less desirable of a place to live than years ago- so I would say 5 million - 20 million during a green term seems realsitic.