If you're dependent
...then your household is the same as your parents' household, which includes:
You
Your parent(s)
Your sibling(s)
Any other dependents that live with you. These live-in dependents must receive more than half (50%) of their financial support from your parents. This may include, for example, extended family like grandparents.
If you're independent
...then for most students (especially high school seniors), your household is probably just you (1 person).
This is because your household (according to the FAFSA®) only includes:
You
Your spouse (if you're married)
Your children (if you have children)
Any live-in dependents who receive at least half (50%) of their financial support from you.
This means that even if you are independent because you have a non-parent legal guardian, you do not count your legal guardians (or other kids in your house) as part of your "household."
For example, if your legal guardians are your aunt and uncle, who also have 2 kids, none of those people count as your spouse/children/dependents, so the number of people you should report "in your household" is 1 (not 5).