Well, we're approaching zero population growth. Basically, in a few generations, the population should be rapidly stagnating.

It will be okay. Just look at India, they're doing fine even though they got as many people as the whole Africa in a much smaller area.

Lack of resources ain't a big problem, it's the lack of utilization of resources in certain parts of the world that is the problem.

With genetic engineering, new crop techniques, extracting salt from sea water etc I'm convinced the earth can hold at least 20 billion people.

Not true. Each woman on Earth gets something like 4 kids on average. That means still a doubling of each generation! Even in the West most couples get on average nearly 2.0 kids (and who is to say that all countries will mimic Western family patterns in the future?).

Even if in the very distant future each woman gets only 1,5 kids on average (very unlikely) that rate of decrease will largely be off-set by a steady increase in life expectancy.

Decreasing population in the West, possibly yes. Decreasing population in the World, not in centuries.

You know what zero population growth is, correct?

A couple having two children on average. The two children replace the two parents. There is a net gain of zero people.

America has accomplished this, along with China (?).

And can you PLEASE get the real statistic of average children per couple instead of pulling it out of a hat?

If we were to build a single city with the population density of Manhattan Island to hold the entire population of the earth it would be (approximately) the size of the state of Massachusetts ... When you start considering high-density food production (100 story hot-houses) the foot-print of man on the earth could be smaller than the state of Texas.

The only resource that limits our ability to grow endlessly is energy ... and it is entirely possibly (and potentially likely) that our ability to produce massive quantities of energy efficiently will dramatically increase over the next couple of generations.

You could also factor in those that die before reproduction - For example, if a woman has 3 kids and one dies prior to procreating another generation.

Actual fertility rate, worldwide is 2.82 - down from 4.95 in 1950. Fertility is projected to drop below replacement (2.33) in 2025.

This thread = People making factual statements without citing a single source.

I'd say the most reasonable population to expect is the kind of figures you've seen in futuristic sci-fi games like Mass Effect and Starcraft (around 10-11 billion).

Whilst this is true for some parts of the world the population still continues to grow for most of it. In a study of 195 countries the population growth was only below zero for 18 countries (source). For much of Europe the population will stagnate, places like Germany and eastern Europe can expect to see their population decline, but some place will expect huge leaps in population by 2050 such as the USA (+100m) and India (+200-300m) (for source see OP).

As for population predictions we can tell that it is beginning to stagnate as you say (Figure 1.). It just depends if it stagnates in the right way, does the population rapidly decline in some areas (like Europe) and continue to grow others (like India) effectively cancelling each other out. I don't know what the answer to that would be though.

Fig 1. World population prediction to 2050 (Source)

While I don't believe in zero growth pop in a long time, I actually believe UN projections are underestimated and that we will pass 10 billion people in this century, I think what you wrote here will offset most of the problems with a growing pop. The potential rate of increased food production is far higher than pop growth, even if we just look at rapidly growing Africa.

And energy yes, it's only a matter of time 'til effective methods for cold fusion or something like that is invented, so in not too many decades from now energy won't be a huge problem at all.

And other vital resources such as metals like iron and copper can be found in the ground. I believe if you just dig hard enough you'll find plenty lol.

No it's not if people live longer. And they do.

Medical tech and genetic engineering will allow those who are born today to live 100 years on average. And for each future generation the life expectancy will just keep increasing far beyond that.