8. If a four-digit number $n$ contains at most two different digits among its four digits, then $n$ is called a "simple four-digit number" (such as 5555 and 3313). Then, the number of simple four-digit numbers is
8. 576.

If the four digits of a four-digit number are all the same, then there are nine such four-digit numbers.

If the four digits of a four-digit number have two different values, the first digit \( a \in \{1,2, \cdots, 9\} \) has 9 possible choices. After choosing \( a \), select \( b \in \{0,1, \cdots, 9\} \) and \( b \neq a \), which gives 9 ways to choose \( b \); for the remaining three positions, each can be filled with \( a \) or \( b \), but they cannot all be filled with \( a \), giving \( 2^3 - 1 = 7 \) ways to fill them. Therefore, the number of four-digit numbers with exactly two different digits is
$$
9 \times 9 \times 7 = 567 \text{ (numbers). }
$$

Thus, the total number of simple four-digit numbers is \( 9 + 567 = 576 \).