You do not give any idea of what clock speed you are thinking of or what its purpose is.
Generally find most folk over estimate the speed they really need.
Moving to a 64 or 80 pin chip means soldering smd parts, are you happy with that - they can be a pain and removal is not easy.
Also most ,if not all, of the 64 pin + chips only have a few ports capable of driving a led directly, unlike a 40 pin 18F.
Don't forget a chips total current handling, between 200-350ma depending of type, with 20 leds you could well overload one chip.
You could use driver chips like the ULN2803, but would sharing the load over 2 18Fs be simpler ??