Count me among the zero reproducers. I think a voluntary limit of two children per two adults would be a great idea. Perhaps a market for reproduction credits, similar to carbon credits, could be arranged.

It does seem to be the case that material security leads for some reason to people having fewer children. So another approach would be to establish material security for all. I have found a way to do that. It can be adopted by any nation.

if curious: "Ending Unemployment and Poverty" (a “4 min read” here in Medium, but not behind the paywall, with links for more about the idea)