Your customers are concerned about S3 storage limitations on some key buckets they are creating. Why should they not be concerned about this?
- There is no limit to the amount of storage for S3.
- They can always create additional buckets.
- There is a bucket maximum size, but there is no limit on the number of buckets.
- AWS can offload additional storage to Dropbox if Dropbox is hosted on AWS.
Explanation
Remember, there is a limit on the number of buckets you can create, and there is a limit to the size of an object, but when taken as a whole - there is no limit to the amount of data you can store in S3.
An Amazon S3 bucket is owned by the AWS account that created it. Bucket ownership is not transferable to another account.
When you create a bucket, you choose its name and the AWS Region to create it in. After you create a bucket, you can't change its name or Region.
When naming a bucket, choose a name that is relevant to you or your business. Avoid using names associated with others. For example, you should avoid using AWS or Amazon in your bucket name.
By default, you can create up to 100 buckets in each of your AWS accounts. If you need additional buckets, you can increase your account bucket limit to a maximum of 1,000 buckets by submitting a service limit increase. There is no difference in performance whether you use many buckets or just a few.
Source
https://docs.aws.amazon.com/AmazonS3/latest/userguide/BucketRestrictions.html