Read this
I wanted to point out why claiming this makes no sense. Sure, I can get into the fact that the manga is named after Naruto, and there is no plot without him. But I'm going to get more technical here. The claim came about because of Sasuke getting so much development lately, but that still doesn't change the fact that Naruto is still in more chapters than Sasuke. Even useless Sakura is in more chapters than him.
Sasuke has had long periods of absence in the manga. In a portion of the Chuunin Exam arc, he was absent for about 10 chapters. I think he might have made one cameo in that time-frame, but that's a bit odd for an arc that he was so important for.
Next up, when Naruto was training with Jiraiya, Sasuke was absent for over 20 chapters. Quite clearly there is focus on Naruto. Naruto hadn't even been absent from the story for that long by this point.
Then comes Sasuke's longest absence, about 70 chapters. Naruto has been absent for maybe 5 chapters in a row by this time. Even Shikamaru hasn't been absent for this long.
After that appearance, Sasuke disappears again for 30 chapters. Sure, what happens in that time does have to do with him, some of it at least. But the focus remains on Akatsuki.
Now we've come to the point that makes people think Sasuke is the main character. Why? Well, he did more stuff in that time-frame. Though there was a point where he was absent for about 15 chapters and Naruto still hasn't been absent for more than 5. And now it's been more than 10 chapters since we've seen Sasuke.
Another argument of the "Sasuke is the real main character club" is that Sasuke drives the plot forward. Isn't the plot: "The Akatsuki want to take all the Biiju and take over the world"? Sasuke only started effecting that about 80 chapters ago. Before then, he really didn't do anything to push the plot forward.
And that's all I have to say about that.