I'm at a small rural private school and teach 4 different subjects, 5 different classes.
I don't think all the sciences count as the same subject. It's still a lot of prep. If I'm teaching balancing equations in chem, that's no use for planning a mitosis lesson in biology and then getting over the concepts of Special relativity in Physics, and then radioactive dating to 2 earth science classes (which are of different grades), so don't progress at the same speed.
I think 5 different classes is a lot. It's 5 different sets of grading, 5 different sets of assessments to create. Also I teach grade 6 to grade 11/12 which requires very different teaching styles.
In my old school, large state comp, I taught 4 classes, 2 honors and 2 standard of the same subject for all, so 4, essentially 2 different courses but Lesson preps were very similar. We didn't always progress at the same speed but similar. My out of class work load was a quarter easy. The curriculum was set out, labs created, general notes provided.
When I came to my new school I had the text books they used but nothing else, no funding either. I had to create the Earth Science Curriculum from scratch.