Keeping a few Linux kernels installed is not a bad idea. If you install a new kernel and for some reason it breaks something else you may have installed or it corrupts itself, having another kernel on hand to revert to can be a huge help. There is a simple way to uninstall unused and undesirable kernels as well as keep the number that will be kept to a lower limit.
To see how many kernels are presently installed on your system.
[jhudgins@p2vm-sys-ppmail ~]$ rpm -q kernel kernel-3.10.0-327.el7.x86_64 kernel-3.10.0-514.16.1.el7.x86_64 kernel-3.10.0-514.21.1.el7.x86_64
The yum-utils package will allow you to quickly remove unnecessary kernels.
[jhudgins@p2vm-sys-ppmail ~]$ sudo yum install yum-utils
I want to always have two working kernels installed on my machines. In order to keep two, I would use the following command to remove all but the two latest kernels.
[jhudgins@p2vm-sys-ppmail ~]$ sudo package-cleanup --oldkernels --count=2
And to verify I only have two installed kernels.
[jhudgins@p2vm-sys-ppmail ~]$ rpm -q kernel kernel-3.10.0-514.16.1.el7.x86_64 kernel-3.10.0-514.21.1.el7.x86_64
To permanently adjust the number of kernels to be installed on your machine you can modify the /etc/yum.conf file as follows.