I need to replace a front strut that is broken.

Since the car is relatively new you could get away with replacing just the one strut as long as it had the same characteristics as the original one. That's what would typically be done at the dealership if the part was being replaced under warranty. Due to the mileage though, I would replace the pair. Even an original equipment new strut won't act the same as an old one. They push together real easily to avoid the tire pushing the body up when it hits a bump in the road, and they pull apart much harder to prevent the tire from dropping down into every hole in the road. Those reactions change with age and miles. Compare that to getting a big hole in one of your shoes. Would it be appropriate to buy one replacement shoe or a pair?

There's no need to replace the upper mounts but you should be prepared for the unexpected expense whenever the struts are being disassembled. Unless they are obviously binding or making noise, the only way to see any roughness when turning them is after they've been taken apart, then the mechanic has to tell you more parts are needed than they included in the estimate. Some shops lure you in with low advertised prices on struts, then surprise you with the added expense after you've committed to the repair and the work has begun. Some shops include new upper mounts in their estimates, then surprise you with a lower final bill if those mounts aren't needed, but they run the risk of losing your business to a shop with that lower estimate. If there's nothing wrong with them now, new struts will have the same wear in another four years. Most upper mounts last much longer than that.