I use Ukuu.
Version 4.17.8 is installed.
Version 4.15.0.23.25 is running.
Whenever I install a new kernel, the 4.15 version always stays.
Is that o.k.?
if you are going to experiment with new kernels it’s always a good idea to keep a kernel known to work installed as a safety fall back. I usually install a newer kernel and keep the former on install fro a week or so just to be sure.
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I do keep 2 kernels because once I had a kernel that caused a kernel panic on bootup. 
It’s won’t hurt anything to have two kernels install as long as you have disc space for them.
With a 2 Tb drive, I have plenty of space.
Automatically the newer kernel should be used whenever you start your system.
2TB is enough but how much to you have set aside for / ? Thats the one that will fill up if you keep too many kernels.
Some encrypted-drive scenarios forces a separate /boot partition and it was rather small causing some issues.
In all Ubuntu derivatives, by default installs, the kernel packages are loaded and updated based on a top-level meta package that calls out all the individual kernel packages like image, headers, extras, etc. It’s not like an app.
How do I know how much is set aside for /?
Spyder have answered my question maybe it might help you as well in your quest
If you installed the system originally with only one Partition you should be all set.
If you install using a separate / and /home partitions depending upon how large you made the / partition or how small that is when the problem can come into play.