In reality, not a lot! Maximum 1-1.5 paragraphs of a 5 paragraph personal statement. Remember you only have limited numbers of characters and you really need to focus on what you love about your subject! A basic structure would be something like this: (tried and tested, both of mine worked to this framework and my Unis liked them!)
1- A show-stopper of an opening paragraph. This is normally the last thing you write, so don't worry if nothing amazing comes into your head when you first sit down at your computer!
2- A "why I love my subject" paragraph. Don't go crazy, just why you are inspired by the field of study and what motivates you to dedicate the next 3 years (at least!) to it.
3- Some more specific examples and case studies from within your field. These can be from your A Level course or further research (which should also appear in this paragraph- the reading, journals etc).
4- Your achievements in your field of study. Could be Maths Challenges, Young Geographer of the Year, creative writing competitions, whatever suits your course. This is an oppertunity to show how awesome you are at your subject.
5- What makes you, you. If you are a team captain, avid dancer, theatre fan, Head Boy, Prefect, volunteer at a local owl sanctuary, tell the Unis. With the exception of Oxbridge really, Unis want to hear about your personality and extra-curricular.