Instances are not equivalent to servers. Just because you have 2000 instances, you don't need 2000 servers to run it. Just like you don't have a single server per WordPress install.

For example, I have 100 Mastodon instances running using 8 VMs at https://masto.host/

I can tell you that a VM with 2vCPUs can run 30 small instances easily (CPU load average under 50%) as long as you have 1GB of RAM for each.

So, if you have a server running, I am sure you can add Mastodon to it and the only thing you will notice is the 1GB of RAM usage. If you want to install it for a small group of users, just find a sharing hosting space like mine or go with a $3 VPS and you will be more than fine.