plot explanation - At which dream level did Inception end?

In the ending of Inception (2010), there is some doubt that Cobb is in the real world. But if he is not in the real world then at which level of the dream he is in?

Starting at the presumed 'real world' as the first level of the story, I think the final scene it's the fifth level (limbo) because at the fourth level they save Fischer, but I need clarification.

Answer

None of the different means you can use to guess whether Cobb is in a dream or not at the end of the movie are completely reliable.

The spinning top has huge issues - the fact that it was Mal's totem, that it appears to act in a simplistic way that perhaps other people could dream, hence invalidating its usefulness - and of course the fact that we are never even shown what the top does at the end of the movie.

I am unconvinced by how completely watertight the wedding ring theory (referenced in @Eoin's answer), as it seems possible for Cobb to dream the wedding ring when he thinks he is in a dream and not otherwise. Yes it is an interesting theory, but whilst it is certainly a deliberate action from the writers/director - the meaning is not completely clear. Cobb clearly seems to treat the spinning top as his totem (flawed or not), as he uses it when flustered by his dream in Mombassa. So yes, it could be a clue, it could even be his true or second totem - but it is still not completely reliable in my opinion.

So is Cobb in a dream at the end? I don't think we can tell. Unlike the other answer, I like the ambiguity in the movie. If pushed to give an answer I would say that the fact that he is such an experienced shared dreamer, it seems likely that he would eventually work out if he is in a dream or not - so I guess he is likely to be in the real world, and the wedding ring therefore might be a true clue to that.

So to answer the original question, if this is not the real world, what level is this? Well if the top level of the story is not the real world, and is in fact another dream, then 'who knows' is the only real answer. The question has no real meaning as there are possibly untold levels of dreaming we have no knowledge of.