Each quarter, I take 3 or 4 classes. 16/17 units is on the high end, 12/13 is on the low end. (I’ve heard that engineers regularly take 19/20 units, which sounds like death to me). I would discourage taking 5 classes in a quarter, unless maybe a couple are super easy pass/no pass or something.
Each quarter you take different classes. As Gumbymom mentioned, these can be in a series. For example, pretty much all science majors seem to take BIS2ABC, CHE2ABC, and a math or physics series.
GEs can be spread out across all 4 years; some people recommend getting them out of the way early, while others recommend saving some for later to alleviate quarters with 4 difficult major classes. Honestly yeah GEs are kind of confusing. I can give you details on how to plan GEs if you’d like.
For an example, here are the classes I took my first year (I’m a BS in College of Ag):
Fall Quarter:
-MAT17A (the first math class for bio majors; engineers take 21A, I think economics people 16A?)
-MUS10 (a GE! I like music and really liked this class btw)
-NAS10 (another GE, kinda boring but not awful; notice that the "10"s in most departments are good for GEs)
-a freshman seminar (only pick one you’re actually interested in, otherwise it feels like a waste of time)
Total = 13 units. I only took 1 major-required class.
Winter Quarter:
-MAT17B (the second math class in my series)
-CHE2A (starting the chem series)
-GEL1 (a major-required class, but a lower-div easy one)
-NAS1 (another GE; this one was really boring btw)
Total = 17 units.
Spring Quarter:
-CHE2B (continuing chem series)
-BIS2A (starting bio series)
-AAS12 (another GE)
Total = 14 units.
So I decided to get my GEs done pretty quick. But you don’t HAVE to. It might be better to wait, actually, because a class you take for your major might unexpectedly fill some GE credits. For example, in junior year I took CMN3 as a major requirement, and realized it filled SocSci GE credits. Basically, don’t worry about it :p. You have plenty of time to fill GEs.