If you are applying for a very different subject you can ask the Uni if they will accept a different PS (if so, then you simply email it to them).
Yes, you apply for only one course/Uni at a time. They consider your application and its only if they say No that you apply to another.
Two important points :
* You MUST meet the stated admissions requirements in terms of subjects/predicted grades and GCSEs etc. This isnt like Clearing whee Unis can be more flexible
* With Extra you wont have an Insurance choice. You'll only have this Extra place as your Firm. So think carefully about the implications of that - ie. that te grades required are achievable.