Limit on Listing User Pools

I'm developing a multi-tenant SaaS application in which each tenant will have a dedicated user pool for its own users. One constraint is that no one can be a root user in more that one user pool. So during user registration, to determine whether an email address is already associated with a root user account, I have to check all users across all user pools. Here's the problem: in Cognito, the listUserPools API has a MaxResults property with an upper limit of 60. If there are 100 user pools in my application, what is the best way to search across all of them given that 60-item search results limit?

Alternatively, is there a way to filter on an email address across all user pools at once, rather than by each user pool individually? I could maintain a separate table of root users, but that seems redundant. Please advise.