Oil came into widespread use around 1900, when the world’s population was approximately 1.5 billion people. A very intelligent man has estimated that the post-peak oil population number would be lower than 1.5 billion people but I respectfully disagree with him.
I estimate that the world’s population in the year 2100 will be 3 billion people based on the following:
(1) High IQ people will spread to sub-Saharan Africa, bringing efficient agriculture with them, increasing the world's carrying capacity.
(2) High IQ people will also saturate the great plains, unlike America-1900 when they were largely undeveloped, increasing the world’s carrying capacity.
(3) Alternative energy, specifically mentioning nuclear power, will increase the world’s carrying capacity beyond what it was in 1900.
(4) Genetics has given us better plants than we had in 1900.
There are excellent deals on fertile land in North Dakota available now.
Graph from Paul Chefurka.