No. All paid plans are billed annually. Payments can be made by credit card or with a PO but in both cases billing is annual.
Secured Resources are top-level objects in the cloud environment. For example, AWS EC2 Instances, Lambda Functions, S3 Buckets, and Load Balancers. The complete list can be found in Aqua Knowledge Base.
It is possible to purchase additional capacity mid-year. Just contact us and we can discuss the next steps.
The minimal paid plan is the Team plan that can secure up to 2,000 resources and can be used on one or more cloud accounts.