“Enough” storage space will vary tremendously from person to person and how they use their devices. I have an original 12.9” iPad Pro, 128Gb with less than 15Gb free. It’s replacement will definitely have whatever is the max. available storage when I buy next.
My suggestion with iPads or iPhones is to always buy the most storage your budget allows. You cannot add storage later, so whatever you buy, you‘re stuck with.
That said, if your budget only allowed for 64Gb and you find yourself tight on space, you may just need to be clever and curcumspect with what you keep on the device at any one time, and make more use of your computer storage, online storage and more frequent swapping of content as needed.