I’m a beginner too, but I’ll try to answer your question anyway (if any person more advanced than I am spots any mistake, please correct me!). It’s a special reading and I don’t think it would have been possible to know with the lesson. I think Koichi put it on the practice page to show that exceptions exist… maybe? Anyway, your “ichinin” would have been good if 一人 weren’t an exception, since 三人 (not an exception) is pronounced “sannin”.

Willy wrote:

So I looked and saw that “人” was “Hito” so assumed “一” would be the “ri”. But I can’t see that anywhere, with “一” I just see “ひと.つ”.

In Japanese, there are many “counters”, “nin” being one of them: to count people. Counters are basically something you put after a number to show what you are counting (the つ you saw in ひと.つ is for counting small objects, for example). When you assumed that 一 would be “ri”, it wasn’t correct. “ri” is actually another way of pronouncing 人, while “hito” is another way of pronouncing 一, as you can see in 一つ (hitotsu) for example.

If we look at 二人, we can see that it’s the same thing: it’s pronounced as “futari”, “futa” being 二 and “ri” being 人.

I hope this explanation helped you a bit and wasn’t too confusing >.<!