Plentiful, plenteous, ample, abundant, copious denote in common more than adequate or sufficient yet not in excess.
Something is plentiful or plenteous of which there is great or rich supply; plenteous is the more bookish term.
Something is ample which is generously sufficient to satisfy a definite requirement.
Something is abundant which is very plentiful or of which there is an unusually large supply. Abundant sometimes implies profusion.
Something is copious which is marked by great abundance; the term is especially applicable when emphasis is to be put on largeness of supply rather than fullness or richness. It is therefore not always interchangeable with the other words. In respect to language copious often implies profusion of words, richness of vocabulary, or fullness of information.