Ancient news by now, but I'm assuming CoroCoro's stage count is a simple mistake. They only reported on the 8/8 Direct and nothing more, it makes no sense that if they did have future info the only thing to get out is a number.
Could there be more than 103 stages? Sure, but I wouldn't be surprised if the game ships with that number.
Always remember that we are definitely getting DLC. Spring Man, Rex and Pyra, Isaac... it's always darkest before the dawn!
I'm honestly surprised we haven't seen any Pushmo content. Mallo has a trophy in 4, you'd think at least a Pushmo stage would be a shoe-in.