Answer: I am surprised ivy would be dying out, it is one of the toughest groundcovers around and is usually difficult to remove from a site where it has been established. Perhaps it suffered some winter damage to the foliage and will rejuvenate as the spring progresses and come back strongly from the roots.
Otherwise, you might want to try to figure out why it is failing before you replant with something else. Along a walk, it might be due to salt damage for instance from deicing salts. This will damage new plantings as well until it has rained enough to leach the salt down lower than the plant roots.
If you decide to replace and use the small size starters plants you would probably use one per square foot, so roughly 60 plants. They are often sold by the flat of 72 so you could use the whole flat there. Periwinkle is a nice groundcover and does especially well in shade or part shade. For a hot sunny location and a slope (which can be dry) you might look at something like creeping phlox, Phlox subulata or creeping thyme. Your local professional nursery staff should be able to help you with suggestions based on a more detailed understanding of the growing conditions along the walk where you want to plant.
Good luck with your groundcover!
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