Every Teacher is Unique
Yes, that means you! And how you utilize Google Classroom can be just as unique.
Should you have a separate Classroom for every course? There is no hard-and-fast rule for this.
Teachers of the younger students might choose to have one classroom (I have personally seen students as young as grade one successfully use Google Classroom) for everything; but separate it into subject material through the use of “topics”. Once a student is able to log on to a Chromebook, utilizing Classroom is a breeze, as it is in their “waffle”.
While the youngest students may not yet be literate, they are so eager to learn. Posting links to websites you use for instruction or learning while at school can be placed in Google Classroom as a “material” and students can find it and show it (or use it) while at home.
As for the older grades…. again; teacher preference. If you teach more than one section of a course, should each have their own Classroom? That’s up to you. If you are going to use (or even potentially use) Classroom for returning grades or setting deadlines, I would recommend a separate Classroom for each classlist. That gives you maximum control of the deadlines, and doesn’t leave your students with any excuses regarding lack of clarity as to when something was (or is) due. Every class can have a different due date.
Some teachers do a lot of grading in Google Classroom (return the grade to students that way), while others do not leverage this feature. If or how you proceed with this is again, your choice.