Before you think about how many advanced highers to take, you should contact a variety of universities regarding how they view them. For example, according to the Edinburgh University entry requirements
"While Advanced Highers, with the exception of Medicine and Veterinary Medicine, are not required for entry, we strongly recommend that you pursue your studies at a more advanced level by taking at least one subject at Advanced Higher. This is because study at Advanced Higher level is excellent preparation for study at Edinburgh. Achievement in Advanced Highers can also contribute to entry requirements for Highers, with Grades B and C at Advanced Higher equating to Grades A and B at Higher respectively. Please note however, that we do not count both the Higher and Advanced Higher in the same subject, for example History at Higher and History at Advanced Higher, in meeting the requirement to have qualifications in at least four subjects at the required grades. Achievement in specific Advanced Highers may allow students to enter certain programmes within the College of Science and Engineering directly to the second year."
Whereas, for certain subjects, Glasgow Uni will double count Highers and Advanced Highers - "Double-counting of Advanced Highers and Highers to meet entry requirements is allowed for Accountancy & Finance and all Arts, Engineering, Science and Social Sciences subjects, and may be accepted for other subjects"
And just to complicate things further, there are also some Scottish university courses that allow you to start in 2nd year if you have good advanced highers.