You can do this - you can have as many accounts with their respective payment details as you want. Then you have a standard 20 instance limit associated with each account.
Keep in mind that these accounts are separate and have their own access keys. Any technology that you deploy to deploy instances in these accounts should be aware of this, as your pool manager will need the appropriate key for the account in which he is trying to run the instance. You may also encounter difficulties with sharing EBS, access to a security group (for example, if you have a domain controller in one account, virtual machines in other accounts cannot see it), and load balancing and VPC are probably will be difficult at best.
Amazon usually responds quickly to requests to increase the limit - because the more copies you use, the more money they make. Obviously, they want to ensure that they have the opportunity for your requirement, so they manually “check” the requests, but provided that you have a reasonable business example that they can easily place.