There are many times the number of people that can sustainably live on earth at the moment (something like an order of magnitude). You can see consequences of this by looking at much more overpopulated areas in the middle east, Africa, or India where starvation is much more a part of life. This is a huge problem in more developed areas as well, with environmental degradation, pollution, and the land made poorer by only a small fraction being wilderness.

A naive idea is, have a baby penalty. A baby penalty would make would be parents think twice about having children, and more about family planning and contraception. It would suck to be a child in a family that cannot afford to raise children (this really needs to be emphasized). To offset this people can be paid to bring up a child that their parents cannot afford, this works great as the people who raise the other parents children are less likely to have children themselves. Now this is naive, is not fleshed out, and has all sorts of things that can go wrong, but conversations about these things has to start. Take China, without their 'Family Planning Policy", the place would be a much worse place to live in than today. What was described is something much softer than the Family Planning Policy, but countries may have to move to something like China's policies to have any hope.

"We are a plague on the Earth. It’s coming home to roost over the next 50 years or so. It’s not just climate change; it’s sheer space, places to grow food for this enormous horde. Either we limit our population growth or the natural world will do it for us, and the natural world is doing it for us right now." - David Attenborough