6. There are 900 three-digit numbers (100, $101, \cdots, 999$). If these three-digit numbers are printed on cards, with one number per card, some cards, when flipped, still show a three-digit number, such as 198, which when flipped becomes 861 (1 is still considered 1 when flipped); some cards do not, such as 531, which when flipped shows $[\varepsilon \subseteq$, so some cards can be used in two ways. Therefore, the maximum number of cards that can be saved is $\qquad$.
6. If the number on the card is reversed, it remains a three-digit number. The tens digit of these numbers can be $0,1,6,8,9$, while the hundreds and units digits can only be $1,6,8,9$. There are a total of $5 \times 4^{2}=80$ such three-digit numbers. However, some of these numbers, when reversed, remain the same three-digit number (such as 619). For these numbers, the tens digit can only be 0, 1, or 8, the hundreds digit can be $1,6,8,9$, and the units digit is then determined accordingly as $1,9,8,6$, totaling $3 \times 4=12$ numbers.
Therefore, the maximum number of cards that can be saved is $\frac{1}{2}(80-12)=34$ cards.