Hey! I ended up doing 13 GCSEs, 2 of them self-studied. It's a lot of work (I did my subjects over about 3 years, with the self-studied subjects being taken in Yr 12), but if it's subjects you're interested in then it's a lot of fun, too! Japanese was one of the GCSEs I self-taught: it's more challenging than Spanish, but not as hard as it sounds. If you work hard and have a passion for the language you can definitely do it!
This is how Oxbridge students do it.