http://www.st-andrews.ac.uk/admissions/ ... greestudy/
That explains most of it.
You will take one Art History module in each of the two semesters. Each module is worth 20 credits, and you must take 60 credits per semester. The remaining 40 credits can be chosen from any subject in the Arts faculty, providing you meet module prerequisites (International Relations, for example, requires certain A Level/Scottish Higher grades). The vast majority of modules are 20 credits, though you will find that some Philosophy modules are 10 credits and second year Psychology modules are 30 credits.
So, you'll be taking 3 subjects in first year - For your first two years, Art History will only be one third of your degree.