Do you need one for left and one for right ear, or just one of either? If just one of either, how do you decide which one to get? On the one hand, if I am rig

Do you need one for left and one for right ear, or just one of either? If just one of either, how do you decide which one to get? On the one hand, if I am right handed, I might find it more easy to quickly take it on and off if it's on the right side. But at the same time, that would also mean that whenever I answer my phone I'd also have to take it out first. Unless it can also be paired with the phone and used when answering calls.