Apparently a lot.
It turns out that just like a computer has RAM (short-term or working memory), we do too. And in this RAM of our brain we can store 7 discrete things.
Of course, that is an average number. But most people can store 7 plus minus 2 things in their working memory. And just to prove that i am not kidding, close your eyes now and reread the last sentence in your head. Got it? Good. Now close your eyes and try to do the same for the sentence before that. Got it? Well, our working memory is limited.
That is unless you practice to improve it.
From Joshua Foer’s Moonwalking with Einstein