Not very often, but it does happen. If you are reasonably good at soldering, it's easy enough to replace them. Are you talking about the plug tip (attached to the cable) or the guts of the jack (built into the amp)?
The cable ends are super easy to replace.
For the jack in the amp, either it is just bent, and can be straightened easily, or it has actually broken off, but that is incredibly rare.
If broken and you have to replace the plug socket, very often it is a standard plug socket that is hand-wired to the board and trivial to replace, and almost any replacement part will do. If it is a PCB-mount socket, you have to get an exact-replacement, and desolder the original from the board, clean the board and mount the new one. Not rocket science, but it can be a bit of a pain unless you are experienced at that.