Keeping at least three “older” kernels is strongly recommended. Sometimes it happens that there are bugs in the kernel that render a device or a function on your computer unusable. If this happens, although not often, but believe me, it happens, you are a happy person if you have an older kernel installed where everything just works.
(FYI, on my systems, I usually increase that number to 5 or 6).
Also, what you need to know; you can remove one or two kernels now but when you get a new kernel as part of the regular update process, the number of kernels on you system keeps building again…
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