How, though? Without more specifics about how the "limit" would work, this statement is fairly meaningless.

(Not to say that I'm necessarily onboard with any single specific way I can conceive of that would establish a "limit" on how many children people can have.)

What's a "couple"? Would governments do something bad (a fine, jail time, etc) to people who had a third child as a punishment/deterrent? If jail time, what would be done with the two kids they ostensibly already have? Would some people who already have two kids be forced on threat of incarceration to undergo abortions and/or sterilization? Maybe all the governments on earth would just make it legal for any person to kill any child with two or more older siblings on sight, hmm? (There's a lot of sarcasm in this paragraph. I hope that's obvious, but maybe it's good to point it out explicitly anyway.)

There is a lot that governments can do to "encourage" a lower birth rate that wouldn't be draconian like throwing people in jail for having kids. Like free birth control, for instance. More funding for womens' healthcare organizations. The word "limit" in your post makes it seem like that's not what you're going for at all.