You need at least two quilt cover sets per bed so that you have a spare set when needed. It is also nice to have a complete set of bed linen for different occasions, such as:
Summer bed linen such as breathable bamboo, linen or lightweight cotton sheets and quilt cover sets.
Luxurious bed linen for nights when extra comfort is needed, such as egyptian cotton sheets and quilt covers in complementary colours.
A heavyweight quilt cover for winter warmth.