I hope they did told you soo when you bought the box.
Cause you'll need to approach your backup in a scaled out thinking way...
Nutanix and Veeam wrote a best practices, but i'm not 100% agreeing with it
I allow my backup to handle multiple vm's per repository but then limit the number of vm's / host that can be handled.
This way i'm sure it's taking a backup of vm's that are spreaded over hosts (read storage with Nutanix) and get some good speeds.
Are you running Hyper-V on the Nutanix or VmWare ?
I run Hyper-v, but advice VmWare.
drop me an email : firstname.lastname@example.org
with your config on both nutanix/veeam/repository environment and what you wish to achieve.
I have almost a year experience now with debugging and optimising this