I’d recommend against underscore as the character is not in RFC952, however minus is. Each HASS instance then broadcasts the name and IPaddress via mDNS / Bonjour as hostname.local. You of course can use an IPv4 or IPv6 address directly.
BAD: home-assistant_1
AOK: home-assistant-1 => http://home-assistant-1.local:8123
AOK: home-assistant-test => http://home-assistant-test.local:8123
AOK: http://192.168.0.222:8123
