The Etch Remover / Marble Polishing product was designed for repairing small spots from spills, glass rings, spray marks, and drips. Making such small repairs does not require any particular skill from the user.
Sometimes people ask about repolishing an entire countertop or even small areas of flooring. It will work the same for a large area, but it's a different matter restoring a small spot (and blending it with the surrounding finish) vs. repolishing an entire countertop. Repairing a small spot is easy and needs no experience. However, for a large area, it can be difficult without significant experience to get an even shine over an entire countertop. Doing so is not up to the product. It is determined by the skill of the person applying the product.
Repolishing marble is like sanding wood. You need to sand the wood evenly over the entire surface in order to achieve a relatively even smoothness. Now if you keep sanding and sanding eventually you will do that as long as you have enough of the same grit sandpaper. Well, marble is similar in that the process is somewhat the same, but simply going over and over and over the surface with this product will not necessarily create an even shine. You have to adjust your application technique from one area to the next to get that even shine.
Now, you may be able to do it or at least do it to the degree it is satisfactory for you. It is not impossible for someone without experience, but you may find it challenging. Again, it isn't a matter of using a different product. This is exactly what you'd use for large or small areas. It's just a matter of quantity and skill of application.
The Etch Remover is meant to be applied by hand in small sections. A power tool with buffing pad may be used, however, you must be very careful or you risk burning swirl marks into the marble. It is best to apply by hand. If you choose to use a power buffer be sure to use minimal pressure, set the speed (RPM) on the lowest setting, take your time, and inspect your results frequently.
Good news is that you won't do any harm trying. If in the end, you need professional help you won't make it any worse than it is now. Marble can always be refinished or repolished. So, no need to be afraid you may permanently ruin your marble countertop. You won't. But if you are able to do it well enough, you'll save a bunch of money on pro help.
Coverage will vary as the severity of etching will vary and the skill of application. The quantity you may need for a large area is a guestimate. One jar of the Etch Remover may cover 5 - 10 square feet. But a truly accurate recommendation is impossible. So, you should get more than you'll need. If you have some left over, no big deal since you'll want some for future etch mark repair anyway.
Also, you'll want the Granite & Marble Cleaning Spray (1 QT) to use in conjunction with the Etch Remover. The Etch Remover is applied to small areas and the buffing is done for short periods. Once you are done buffing one application, it is important to neutralize the Etch Remover and clean it off the surface immediately. You can't allow it to sit on the surface without buffing it.
The Etch Remover works by softening the marble so the abrasives it contains can work to repolish the surface (smooth it to a shine) with rubbing / buffing. If you leave it on the surface, the softening action will leave the surface dull. Not permanently, but it's two steps forward and one step back if you allow it to sit too long without buffing or cleaning it off. Multiple short applications are usually needed to completely restore the shine.
The cleaner is used to help neutralize the softening action and then clean the surface to remove all used Etch Remover. Once the repair is completed, you can then use this cleaner as your regular cleaner.
Hiring a stone restoration professional is often the best choice for repolishing larger areas, entire countertops, flooring, etc. But with patience and maybe a little trial and error you can produce the desired results on large areas.