Help me understand Sheets pricing
The pricing plans are available here:

You will see Google Sheets Read/Write Operations under each plan. This is how many times you can use the Sheets widget(s) in a month. One use event occurs each time you save a distinct data item to Google Sheets or retrieve a data item from Google Sheets. For example, in a scenario where your course contains 5 quiz questions, and you save the response to each question to Google Sheets: a learner who takes the course one time and responds to each question one time will use 5 credits. If you have 10 learners answering 5 questions each - you will use 5x10=50 credits.

You can track your current usage and your remaining credits on this page:

There is a default email notification that goes out when you exhaust 80% of the credits for the month, so this will give you enough time to make arrangements if needed. You can choose to upgrade to the next plan if you’d like to keep accessing Google Sheets from your courses or do nothing. If you run out of credits, you will not be able to use the Sheets widget for the remainder of the billing period, and the credits will reset with the next billing cycle. You can verify that the notification is turned on (or turn it off if needed) on the Account page.