All we need is more subsidized irresponsible reproduction
I don't think we're likely to see too many limits in the near future on what number of people we can support though. We can purify seawater if we had water shortages. There are plenty of available energy sources we aren't using and food ... well it's not impossible to even have a green housing the desert and recycle water vapor if people were desperate enough. Hopefully we'll tet into space eventually and then at least if something major did happen, it wouldn't be the end.
You can tell how difficult it is to increase population merely by looking at the cost of raising children. Many places it's just too difficult to have more than a child or two, so people refrain. I think expecting everyone to pay for others reproductive irresponsibility is a mistake though and only aggravates the problem.