We have a production environment with over 100 Nintex DB’s and the only problem has been
manageability. There are also some boundaries in SQL server regarding number of
databases, specially consider this if you are planning to run Always on availability
groups.    

32,767 are the number of DBs per single instance. The other limitation that has more potential to impact a one-to-one mapping of Nintex and SP databases is the 500 supported limits per web application. Over 500 administrative tasks slow.

Nintex Support has assured me that there are no performance issues related to the number of databases. To quote:

"There is no negative impact on performance that can come from multiple Nintex Content databases."

Why not just release the load testing that, hopefully, has been done on the product?

We use one SQL and one Nintex per client. This makes backup and restore simpler and our naming convention makes it clear how they are paired up.

One consideration when choosing how to setup the SP/Nintex pairing, are the SharePoint workflow limitations and boundaries​. Actually there are several there. The one that is a consideration for my architecture is that the number of workflows (15) processing is a per content database setting. This is not a hard number but adjusting this requires other factors to be taken into consideration. since it is a global setting. This article is based on 2007 but should provide a good starting point for looking into those other factors.

Check out this excellent deep dive into SharePoint Workflow Architecture​