Mistake #2 Buying insufficient amounts of a single variety
Solution: "Taking home only one or two pots of a particular specimen is a surefire way to make your garden look hodgepodge," says Flora. Depending on the size of the plant, she suggests buying at least three containers (though six or more of a species is preferred) for a unified well-designed look.
Now I know what your thinking....how do I know this is going to be a plant I want to buy that much of. Sure it's nice but do I want a dozen of them. Well, that's when patience comes in. If your not sure if the plant will perform well in your spot then buy only one or two and see what happens. If you like it then do more the next year. Just remember to keep the tag or write it down in a garden journal. Also make sure that what the tag says is really that variety. Ask the nursery manager or owner. Sometimes tags get removed accidentally.
Another point about mass plantings. It may not be the look your after. There's something to be said for the cottage garden where it is a hodgepodge of different plants ~the true plant lovers garden. If your a gardener that spends a good deal of time in your garden your plants will probably look good even if there's only one of each kind.
If you want a more formal or professional looking garden then mass planting will do that. It's also for the homeowner who doesn't want to spend all their spare time in their yard or in the nursery looking for the latest plant, but just wants it to look good and be easier to maintain.
Hope this was helpful....
take care and be kind to the earth.