You can easily run multiple Gluetun containers on the same host machine without any conflict.
On the other hand, there are few tricks to lower resource usage.
You can run a shared DNS server for all your Gluetun instances.
You can have a DNS server such as qmcgaw/dns:v2.0.0-beta and run it in the same Docker network as your other Gluetun containers.
You can have it non-VPN'ed or VPN'ed through one of the Gluetun containers.