It’s always a good idea to have a second trunk from another provider to use as a backup in case your main provider goes down or has issues.
Generally, a trunk can handle as many calls (channels) as the provider will allow. Most providers claim to allow an unlimited number of calls, but actually have some kind of undisclosed limit for security purposes. So, you generally don’t need a second trunk just to get a second call
If you have a trunk that allows unlimited minutes, then your provider will likely have a restriction on the number of channels, but you can often increase the number of channels by paying more money without having to set-up a second trunk. I believe that’s true with SipStation.