Three is feasible. Any more than that and I would be sceptical. I would be interested in hearing that person's story. It's a very grey area though because someone can speak a language well, but may not know how to write it, as you allude to. Then there are people such as my friend, who moved to the UK when he was a kid from Somalia. English is without doubt his strongest language. But it's not his native language, that would be Somali. He didn't start speaking English until he was 10 or 11 years old. What would he put?
I definitely don't think listing more than one native language means they have to be equally strong. Nor do I think it means they have to have a C2 level. There are different levels of mastery I guess. Millions of native English speakers would fail a C2 English exam, simply because they have no cause to know the material. But English is their native language. And they would have knowledge of informal/dialectal language that a non-native speaker at C2 level may not know, and indeed may not ever know.
I know someone who has three native languages. His family is Catalan/French and he consequently grew up speaking French and Catalan. But he has lived most of his life in Catalonia, so he knows Spanish as well. He can speak all three languages like a native. But even so, I'm pretty sure Catalan is his strongest language. Then there's the fact that these are three closely related languages linguistically speaking, it's not a stretch to believe someone has a native grasp of all three.
I know of another couple, the father is Danish and the mother is Russian and they are raising the children in the UK. They speak all three languages at home, but the children attend special classes for the native languages of their parents. It makes sense, without that they would probably be able to speak and understand the languages but not be able to read or write them very well. It's a complicated subject.
Thanks a lot, Vera and Paul, for your comments!
Yes, it's a complicated topic. Somehow on italki I expect someone to have at least C2 level, when I see the green bars, but I know that in reality, it is sometimes difficult to define which language is the actual mother tongue. And I totally agree with Paul, that three is feasible, but I guess, you can speak three languages native-like, but it would be difficult to be able to write them with the same amount of literacy.
It just bugs me a bit, when I see people on italki, claiming to be native speakers and offering help with this particular language, but clearly aren't able to write it properly.
Well, judging from his tweets and speeches, even the current Potus wouldn't pass a C2 test, but then the conclusion is, that native speakers aren't automatically the best language partners and teachers. Many users here specifically look for native speakers, but I think a non-native speaker with C2 level could be a better help than a native speaker with B2 level when it comes to grammar or spelling.
I, as a language learner and teacher, do care a great deal about grammar. How do you learn languages without caring about the grammar?
Vladana, my mother tounge is Croatian and I can understand Serbian, Bosnian and Montenegrian perfectly (95-100%) but everybody will notice when I start to speak (or write) their language that I'm not native, there are some differences between all of them. So if I understand it well but I don't speak perfectly, it's hard for me even to tell which level I am in that languages (I don't have it on my profile because of that).
I noticed on italki that here is an inflation of "native" speakers (users but also tutors/teachers) and i I wonder if they really like to learn (or - even worse - teach) foreign languages. For me it's really misleading if someone tells he is native and he is hardly C1... they obviously think they can find a language partner or students if they pretend to be a native speaker?!?
italki is a great place for learning a foreign language and for me it's really motivating when I see that someone who is native is really mastering his/her mother language but on the other hand it's also really great to see someone who is proficient in their foreign language (C2), they are a living proof to me that I can master that language too if I put effort and time in learning.