On the one hand, as capital de-structures traditional ways of life, it deprives large masses of their former resources and only integrates part of those masses into wage labour. On the other hand, as it continually substitutes capital for labour, it periodically fires a portion of those it has hired. Where the system is the most dynamic, increased production and the creation of new sectors absorb the labour surplus. Elsewhere, or whenever capital stops investing in its heartland, millions of men and women are unwanted, all the more so as medical progress lowers infant mortality and increases life expectancy.
There's no natural limit beyond which over-numerous human beings would inevitably end up in antagonism and mutual destruction. The problem is the ratio between a society's demographic growth and its ability to integrate newcomers, whether they come from rural areas of the same country, or from abroad, not forgetting that resources are expandable but not infinite, and that social structures are now getting tougher. In about a century, world population has gone from 1,5 to 6 billions, and might reach 9 billions between 2050 and 2100, but figures only matter in relation to why and how we live: 6 or 9 billions will neither feed themselves by roaming the land with a digging stick, or by shopping in drive-in hypermarkets.
Human densification is relative. A few thousand people could be over-numerous in a desert, while millions peacefully rub shoulders in a fertile plain. Still, cramming together huge impoverished crowds develops diseases and epidemics that find favourable ground in mass travelling caused by business, labour migration, political disorder and tourism. The losses due to AIDS in Africa are well documented, with as much as one third of the population being HIV-positive in some countries. This reality is social: vulnerable populations are more prone to carry viruses and spread diseases. After 1914-18, the Spanish flu pandemic killed 25 million people in a few months. Demographic growth goes together with socio-economic conditions that weaken people and enhance inequality. Better go to a hospital in Dallas than Nairobi, unless you're poor in Dallas and rich in Nairobi. But there are limits to the degree of inequality a system can stand. Too many poor or dead, too much ecological and human damage is detrimental to profitability. Capitalism is hardly compatible with chaos: struggle for life and survival of the fittest are more favourable to the enrichment of a few than to the expansion of wage labour. It won't do to have population growth regulated mainly by famine, epidemics and war (several million dead in the Congo in the last ten years).