In the United States at least, the population could be much, much higher than it is and still be supported. Theoretically, we should have about the same population as China if you compare arable land. This can be seen in just how little of the food we use we actually need, with most being exported.
Yeah, look at North Dakota's population history to see how true that is. Those counties in Montana and North Dakota didn't empty out because they couldn't support the farming, they emptied out because the farmers couldn't make a profit off of things. That is, the government could have pursued a different agricultural policy to keep people there.
Not even getting into Northern Canada (south of 60N), which can support a bigger population given economic reason. From what I know, the Abitibi region in Quebec is an example of what can be done with colonisation of what was later called "Mid-Canada". Go look at the proposals for settling Mid-Canada as well, some of which are outlandish and although intended to counter the Soviet Union, actually would only have been possible with a Soviet-style totalitarian government
We're roughly at max pop without resorting to outlandish shit just yet
What you need is to redistribute the population
Nope, we have enough food and can make even more if needed. We have enough resources if we choose to produce/mine them to keep costs down. The issue is people wanting to live like Westerners and the associated environmental cost of producing the resources and food needed for that.
If everyone lived as serfs or whatever to a small class of global elites, then you could have easily have 15-20 billion people and environmental issues no worse than OTL in 2017. That might be a bit ASB, given how this global elite might rise, which somehow eliminates the middle class--like the scientists/engineers inventing shit and geologists finding these new resource deposits-- in the process.