Q: Can I access/manage multiple remote computers at different IP addresses across the internet from one location using Apple Remote Desktop at one time?

Yes this is possible as long as you know the external IP addresses; configured Remote Management on the computers you want to control and - more importantly - configured the Firewall at those sites to port forward the relevant ports to the desired internal IP address or you've configured VPN access at those sites if you don't want to use the port forwarding method.

The specifics on port forwarding and VPN configuration will vary depending on whatever is being used at the network edge of those sites

Excellent. I'm 60 miles away from our main site that my boss wants me to maintain 3 Macs. Setting up VPN later this week or next.