I see it as two different questions, and I don't see it as moral to control folks' free movement around the world via political borders within which people engineer via social agreements "how many people they want to 'allow.'"

We just can´t have that intelligent conversation. I share your frustration entirely.

The 1970 population of 200 million was a nice round population, as opposed to the bloated 300 million today. The question really isn't "How large a population should we have" but rather "How can we get rid of the surplus?"

Getting rid of the surplus is a topic which should not be pursued in public (lest the plans leak out and people start rioting). Instead, let's talk about the problem of "too many people":

Americans use a lot more energy than other people do. In a time of perilous global warming, it is essential we sharply and permanently reduce our carbon output. Even with all sorts of green energy, we still consume too much energy and produce too much waste. So, from now on, Negative Population Growth. That means the following:

Illegal immigrants and non-citizens will be expelled in the direction of whence they came. Since the average Honduran uses less energy than the average American (even a green American), the earth will be better off with all Hondurans living in Honduras. Getting rid of illegals reduces the population from about 320 million to at least 300 million in one fell swoop. (Of course we can send them back whence they came from. It will merely be a complicated logistical problem and people will be quite annoyed about it, but... tough.) Next, NPG: Negative population growth. Breeding pairs can be discouraged from doing what comes naturally,. (Well, they can do it, but take a pill, get those vas deferens snipped, tubes tied, what ever it takes...) We need to get the birth rate down to a population shrinking level.

Men and women should be encouraged (taxation, financial incentives, political reeducation camps in North Dakota, Mississippi, and West Virginia...) to have their single child while they are young and healthy.

Eventually the population will get back to around 200,000,000.

Homosexuality will be incentivized.

People will be encouraged to move to states with unpleasant weather which discourages exuberant sexuality. Florida without AC, Minnesota without heat, that sort of thing. There are also many cities which are so depressing people do not reproduce there. Gary, Indiana, Akron, Ohio, Bakersfield, CA, Las Vegas without gambling or water, etc. Living in depressing towns will be incentivized.

"Other methods" may have to be employed... Never mind what. You'll find out if you are not careful!

How many people do we want to have in America? — Jake

Picking a number would be arbitrary.

We should want continued prosperity, which depends on economic growth, and economic growth depends on population growth to provide workforce and consumption.

Is it conceivable that no controls on migration would create some kind of migratory entropy in which no one nation was significantly more desirable to live in than any other?

If the answer is no, then putting aside all the cosmopolitan arguments and accepting that a nation state is an administrative region, we can ask OP's question in a slightly different way:

How much immigration can a nation state take before experiencing unacceptable negative administrative effects?

Clearly, some nations are more attractive to migrants than others. To what extent can that attraction be compromised before an immigration policy becomes self-defeating?

One can be pro-immigration and accept that there have to be limits to immigration. The difficult questions arise in defining these quantitive and qualitative limitations.

An open door policy may be a good thing up to the point that it isn't anymore. Migration is dynamic and what is acceptable one day may not be acceptable the next, which is to say that there is not a simple and unqualified philosophical position that one can take on immigration that doesn't take into account the contingent effects of immigration on a nation state.

We should want continued prosperity, which depends on economic growth, and economic growth depends on population growth to provide workforce and consumption. — Relativist

Ok, so the answer is 17 billion. Thanks.

Men and women should be encouraged (taxation, financial incentives, political reeducation camps in North Dakota, Mississippi, and West Virginia...) to have their single child while they are young and healthy. — Bitter Crank

Nonsense (or was this a bit tongue in cheek, bitter?). Everywhere they have had such ludicrous policies they have only backfired creating a bigger mess. The simple answer is and has been everywhere: make people more affluent and they will have less children. That's it.

We could easily accommodate far more people. What needs to change is how we engineer living spaces, food production and distribution, transportation, etc.

We could easily accommodate far more people. — Terrapin Station

But do we want to?

You're assuming there's a specific goal or something with allowing immigration. It's just weird man. We might well have broad things we want to continue but not a specific end point we seek beyond which we say "Don't want". It's not a question of space concerns (plenty of room) nor of money (plenty of that, though largely concentrated among the hyper wealthy).

I mean it's a bit like how we have rights. We don't keep free speech until we reach some point where we take it away. Rather, having it (or in this case, allowing immigration into the country) is something we seek to maintain because of some set of principles, not as a goal-oriented policy. Some people construe it that way sure but that's not why most people are fine with it (depending on circumstance).

Here's an example from Florida (now 7 times more people than when I was born). Along the coasts especially the roads are clogged and there's no way to widen many of them further without knocking down trillions of dollars worth of buildings.

Incentivize people to move elsewhere in the country. I left Dallas because the traffic is stupid in certain areas and it was costing me money and time.

I mean I know I'm probably making this sound easier than it is but then you're talking about whether or not we should continue to allow immigration which is way more complicated and tenuous than what I'm saying.

Immigration law and policy is a status quo issue.

All nations limit entry. This suggests there is nothing wrong with it, although on its face it is a populorum justification.

Unlimited entry creates a security issue and also a burden on the taxpayers issue. This suggests that limited entry is justifiable on the basis of the welfare of the citizens. So I will go with that as a good justification.

Numbers of new entrants is a macroeconomic issue, plain and simple. How many new entrants can a national economy afford to accept? This is just a number crunching issue.

One cannot discuss a population goal for a country without discussing a population goal for the whole planet(Currently at around 7.6 billion). Seeing how survival of a species is directly linked to its reproduction habits we start moving more towards eugenics then immigration.