A ribbon isn't that expensive to buy and test if you already have a functional typewriter. The ribbons I use and sell are capable of producing 100+ pages. There are also procedures for significantly prolonging the life of a ribbon, albeit print quality may suffer as a result. The short answer is that you can't really estimate how many pages you can write with a typewriter ribbon because it's not a fixed quantity like sheets of paper. A single sheet of paper can contain from 1 to 20 pages, depending on what kind of paper it is. With a typewriter ribbon, this becomes even more complicated since each individual letter produces a different amount of ink on the paper. When you write with a pen or pencil, you usually try to keep track of how much space you've used by counting lines or pages. But with a typewriter ribbon, there are no visible signs that it's run out of ink until you reach the end of the document where all the letters stop printing. To make sure you don't run out of ink before the end of your document, you need to check how much space you've used every time you start a new page. You do this by looking at the area of the paper that has been printed so far. The more space you can print without running out of ink, the more pages you can write.