Replacing one bad strut on a car is okay if that's all you can afford to replace right now.
But if you do only replace one strut on a car then you should save the money and at least 6 months to a year later you should replace the other strut.
The best thing to do though to prevent problems with the cars suspension is to replace both struts at the same time so that you can then realign the wheels properly.
That will make the vehicle ride better and also make the tires wear out evenly instead of unevenly.
When you do replace the struts you really should have the wheels aligned to be sure that the wheels are in alignment to prevent your tires from wearing out.
I have replaced one strut before when I was poor and could only afford one strut at a time and it worked okay but eventually I replaced the other strut because the ride was a little rough still with the other bad strut.