Hi there!
Thanks for reaching out! I will try to help with your questions.
You can use the same Aggregator license on up to 10 sites. It is limited by how many imports you perform each day, and the limit would be shared across all the sites you use that license on.
The Events Calendar Pro plugin in not required for using Aggregator. You can check out the benefits that the Pro plugin provides by visiting its product page.
Hope that helps! Please let us know if you have any further questions.
Thanks,
Sky