What is a reverb bus?
This will be your reverb bus. A bus will let you send multiple tracks to a single reverb process. Once you have your bus set up, route all the tracks that you want hanging out in the same space to it. Apply some subtle reverb and tweak to your taste.
Should you always bus reverb?
You can apply any and every effect to a bus just like any other track on your mixer. Nothing is more important to a song than the instruments themselves. This means the reverb should never get in the way of anything.
How many reverbs are in a mix?
It's ok to use more than two reverbs, but try to keep the number pretty low. You can go really simple and just use one mono room reverb. I don't recommend doing this for every mix. But it's helpful if you want to create depth in your mix without doing much to change the overall space.