You'll notice that numbers 4, 7, and 9 have two options. Japanese speakers use either, but some are more common in different contexts. For example く is common in months (九月「くがつ」 = September), and しち appears in dates (七月 「しちがつ」 = July). Knowing both is important.
Just like most languages, 1–10 are essential. But unlike English, there are no unique words for eleven, twelve, twenty, thirty, etc. Instead, Japanese builds numbers in formulas:
Formula for 11–19: じゅう + [ones digit]
Formula for 20–99: [tens digit] + じゅう + [ones digit]
To make eleven, you combine "ten" + "one" →
To make twenty, you combine "two" + "ten" →
To make twenty-one, you combine "two" + "ten" + "one" →
To make sixty-nine, you combine "six" + "ten" + "nine" →