As you might or might not be aware, Japanese has many different counters for counting different things. It’s not the same as in English. Most people agree that the best way to remember counters is to learn them straight up.
When counting people, the counter usually ends with にん (for example, to say three people 三人 = さんにん), but for the first two it’s slightly different. 一人 = ひとり and 二人 = ふたり.
When you learn vocab in the early stages of Textfugu, Koichi doesn’t state this which leads to a lot of confusion for some people.
Fortunately, there’s a good guide on Tofugu which teaches you some of the most popular counters. There’s more out there though.
http://www.tofugu.com/guides/japanese-counters-guide/