If you are an exchange student that is going to a university in Europe for student mobility, then you may be faced with something called ECTS which stands for European Credit Transfer System. Basically, European Universities use ECTS to denote the credits for the courses offered. Roughly 25 hours of coursework would equal 1 ECTS compared to 40-50 hours of coursework that is required for 1 US credit. Hence, in general 1 US Credit can equal to 1 ECTS credit in a European University.
In general, most students will be allowed to take 30 ECTS credits of semester courses when they come for a student exchange in a European University. If you are going for an annual student mobility, then you will be expected to study about 60 ECTS in one academic year. As credit transfer policies may differ from university to university, it is suggested that you contact your international student office at your home university.