Hmm, I must say that in my point of view it doesn’t really matter how many immigrants a country has in order to make it a nation. People have changed places, nations rise and fall all throughout history. Just take a look in the timeline and you will at some point find out that anybody on this planet is an “immigrant” somehow. So this whole question about how much immigration a country can take imho breaks down to the question about how to deal with them.
And I say: Integrate them and they will be as good as anybody else!
If you do not integrate people they will not contribute to society, no matter what their passport says, even when they are from your own country.

My 2 cents about cultural identity: Culture, traditions, values, all this doesn’t get “destroyed” by too much integration. It just changes. We may not like the way it changes but that is it, basically. And this is the point people refer to “destroying” a nation: changing it in a way one does not like it to be. (Ok, maybe apart from the very direct and physical way of bombing it to the ground..but lets leave that aside for this debate.)
Here we enter the stage of democracy and how much it must take care of the issues of minorities – but that is another thing.

Hence: Welcome millions of immigrants, integrate them well and there will not be a problem. Let a few tenthousands in, leave them all alone and you will have a lot to deal with in your future…
(I will not specify what “integrate them well” means because this is highly delicate and time consuming — hmm, ok, maybe in another debate thread (; )