When you assign Microsoft Forms quizzes to your students in Teams, these Forms don't affect your individual limits.
How it works
Assign a Forms assessment to your students using Assignments in Microsoft Teams:
A copy of that Form then saves to http://forms.office.com under the Group forms tab. Select the class where you assigned the quiz, and you'll see your Form.
Since this Form is now saved under your Group forms, it will count towards your Group form limit (200) and not your personal Form limit (200).