I came across a post on the GW houseplant forum asking how many multiples of the same plant did people have in their collections. Off of the top of my head I have:
Cacti/Succulents
Adenium Obesum: 8
Euphorbia Obesa: 6
Ceropegia Woodii: 2
Echeveria: 6
Gasteria: 3
Schlumbergera: 5
Orchid Cactus: 2
Other Houseplants
Amaryllis bulbs: 9
African Violet: 3
Perennials/ Bulbs
Allium: 2 (Schubertii, Cernum)
Tulips: 3 (Queen of Night, Flaming Parrot and various X)
Echinacea: 4 (White Swan, Magnus, Tom Thumb, Double Decker)
Heuchera: 2 (Crimson Curls, Heuchera X 'Crown Jewels')
Hemerocallis: 3 ('Fairy Tale Pink' 'Siloam Fairy Tale' and various X)
Tender Bulbs/tubers
Gloriosa Superba: 4
Zantedeschia: 6
Colocasia: 5
Vood Lily: 6
I'm sure there are more but I can't think of many now. Are there some plants that you really like and have more than one or one variety growing?