Usage limits may vary on your QuickBooks Online (QBO) subscription. Let me provide the number of classes you can create depending on your subscription.
If you're using the QBO Simple Start and Essentials versions, the option to create classes isn't available for this subscription. However, if you subscribe to the QBO Plus version, you can set up 40 combined classes and locations. And also, with QBO Advanced version, there's no limitation on creating them.