What is the anime where a popular soft drink kills people who drink it X times?
You may be thinking of an old Miyazaki movie called Sora Tobu Yureisen. In the movie, there was a drink called "Boa Juice" which is manufactured by the evil corporation and was really some kind of poison that made people dissolve if you drank enough of it. 



This blog post has a really detailed synopsis with lots of screenshots of the movie.