I think it would depend on how large of an area you want to fill, and how quickly the particular daylily(s) increase.
I had a relatively large area that I wanted to fill with one cultivar so I bought 6 of that cultivar. After 2 years I split them and had 12. Another two years have 24 and now the area is full. So it took me 4 years and 6 plants.
If the daylily didn't increase quickly I would have had to start with more plants, or just be willing to wait longer.
I would guess the vast majority of people just buy double-fans of lots of different daylilies. If you want to fill an area with just one or two different cultivars you'd have to buy more than just one double-fan of each or it may take years to fill in an area...
