Ubuntu will offer you new kernel versions for update in different contexts:
- New minor releases of the current kernel, if there are security vulnerabilities patched or new features (e.g. a new ABI version)
- If you have activated HWE kernels, then new major kernel versions as they become available (typically from newer Ubuntu releases)
In addition to that, Ubuntu (as most distros) will keep at least one older version of the kernel around, after a new one is installed: If the new kernel makes problems, you can easily revert to the tested version.
apt autoremove you can clean up all your no longer needed kernel images.