In this class, you may learn a lot, but if you are not interested in this class, you may select more other classes in this course.

i think that it is a general question, you should be more specific

class is singular(just one class)

classes is plural (more than one class) 

I don't understand what specifically you're confused about. You're probably overthinking it. 

Just as with most other countable nouns, adding an '-s' or '-es' to a word makes it plural.

So, if you had just 1 class, you should say, "I went to class."  If you had two or more classes, you can say, "I went to (my) classes."