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See it as grouping. For example you will have a high level diagram that consist of a subject boundary and approx 5 packages.

The packages are the subject bounaries of your next diagram etc etc.

I generally use as rule not more than 5 use cases or packages in a subject boundary.

So how many levels? Well that is up to you, it depends what business process you describe aslong levels do not dictate sequence (solution) you be ok.
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