11. Choose three different digits from $0,1, \cdots, 9$ to form a four-digit number (one of the digits can appear twice), such as 5 224. Then the total number of such four-digit numbers is $\qquad$.
11. 3888 .

Discuss in three scenarios.
Four-digit numbers that do not contain 0:
$$
\mathrm{C}_{9}^{3} \mathrm{C}_{3}^{1} \mathrm{C}_{4}^{2} \mathrm{~A}_{2}^{2}=3024 \text { (numbers); }
$$

Four-digit numbers that contain 0 and 0 appears only once:
$$
\mathrm{C}_{9}^{2} \mathrm{C}_{2}^{1} \times 3 \times 3=648 \text { (numbers); }
$$

Four-digit numbers that contain 0 and 0 appears twice:
$$
\mathrm{C}_{9}^{2} \times 3 \mathrm{~A}_{2}^{2}=216 \text { (numbers). }
$$

Thus, there are a total of $3024+648+216=3888$ numbers.