I attended a pro hockey game for the first time since I was a kid, and was wondering when players know when to sub in and out? It seems far too chaotic for a coach to shout someone's name for a one- or two-player substitution. The whole line changes seem a bit easier to picture.
The lines should be set and known before they get on the ice. The coach will have to do some shuffling during a game if a player becomes unavailable. But otherwise they can just call out a line and the players know who is expected.
During play, the coach will tell the bench which line should be ready to go out next. If there's no stoppage, the players on the next line have to be ready and watching for their corresponding teammate to come out. Without a whistle, the line on the ice will have to get the puck into a safe situation to change (not right in front of your goal).
The coach doesn't call the timing. The players need to practice how to play hard and then come off the ice before they're completely spent (usually between 40-50 seconds for NHL teams). The period of time they're on the ice is a "shift".
If the timing is right and they get the puck safe, they'll break for the bench. It's the next line's responsibility to get on the ice properly as players approach. There are different rules for the procedure at different levels.
If done right, all the line players swap within a few seconds and everyone on the ice is fresh. There's lots of ways to mess it up. You can have a player not pay attention and miss that the change. Then if the puck comes back the player can get "trapped" on the ice and completely spent. Being too casual with the puck when starting a change is bad as well.
The coach isn't going to be calling for individual subs. If there's a problem, they'll change up the lines while players are on the bench.