Keep your eyes open for a sheet of newspaper that has a full page ad that has some writing, but a large white space. When you're born, your brain already has a lot of information stored in it, represented by the writing on the page. But there's also a lot of room to learn more. That's the white space on the paper - the brain space that's left to "write" on.
It's difficult to put numbers on how "full" one's brain is at birth and how much "room" there is for future learning, as the more you learn the more connections your brain makes. It appears that there's always room for more learning to take place and that the brain is never "full" (to our current knowledge).