Migrating multi-tenant application to Cognito ... how many user pools is too many?

I have a multi-tenant application that we are wanting to migrate to Cognito. The current system has unique usernames for each tenant but no unique requirement between tenants, with the login screen asking for the username and the tenant name. The initial impression I get from looking through Cognito documentation is that we should use separate user pools per tenant. However, we have over 11,000 tenants each containing a small handful of users. This feels like we would be scaling in the wrong dimension, but because of the username collision I'm not sure there is a better solution. I thought also about making the Cognito username a composite of the existing username and the tenant id, but I don't want that architecture decision to come back to haunt me down the road if we decided to expand and use the hosted UI or add federated/OTP login flows once we are in Cognito. Would love to hear others' thoughts on the best way to handle this situation.