What will the worldwide population be in the year 2100??

Since, currently, the entire population of the world can fit comfortably in the U.S. I don't think you have to worry. To quote 'Pete's Dragon' "There's room for everyone in this world, if everyone would make some room."

the population could be anything from billions to 0. depends whether God Almighty comes to take His servants home.

After the plagues, the two nuclear wars, and the revolt of the mutants, maybe 3 or 4.

If the human race do not try to wipe each other out!! and if we do not have a nuclear war! the World could be overpopulated.

I'd forecast around 5 billion. Once the people of any country reach the fairly low level of development at which it's economically advantageous to have a few educated children rather than many uneducated ones, population growth falls off sharply over just one generation. People with ten siblings typically have only two or three children themselves. At a later stage of economic development, reproduction rates fall below the level necessary to maintain population, as is already happening in many parts of Europe. So Western population as a whole will decline during this century and much of Asia will be following suit before the century ends. BTW, 93 years is not two generations.... more like 3+ even at the present average age of first childbirth in the West.