One sub account for science (department) is probably enough.
Canvas puts all courses in the root account by default, so the only purpose for sub accounts is organization. You can choose the organizational structure based on what makes sense for your setup or size. We have over 350 schools, each school has a sub account, each school has sub accounts for SIS courses vs Manual Courses for sandboxing, workrooms, content, etc. The SIS Course sub accounts are configured so that when we import courses from the SIS the courses go into the sub account by department including Math, Science, English, Foreign Language. Math does not contain Algebra vs Calculus. This structure allows the department chair to be an 'admin' at that level but no level above it. It allows Principals to be the admin of their School but not above it. It also allows us to create sub accounts for departments or divisions.
The biggest consideration for sub accounts probably comes down to naming convention and logic. If you are going to be using SIS Imports and automation (or even spreadsheet) you'll want to make sure the SIS ID for the account identifies the structure, so the course will get to the right sub account, especially if you have multiple or potential for multiple schools.
If you only have 1 school this isn't such a big deal.
Ours looks like this...
SIS Import Format Documentation - Canvas LMS REST API Documentation
Admin Tray - Sub Account Menu
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