You can have as many sub-levels as you like but we recommend not using more than 3-4 sub-levels as it makes it difficult to work with (especially on mobile devices).
Sub tasks are a great way to break down a complex project or task into smaller bits. E.g. the task: Find a moving company can be broken into:
- Do an internet search for moving companies
- Send your request to 3-5 companies
- Evaluate each company