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 shows $86 \mathrm{I}$ (1 is still considered 1 when flipped); some cards do not, such as 531, which when flipped shows IES. Therefore, some cards can be used in two ways, so the maximum number of cards that can be printed is.
34
6. 【Analysis and Solution】If the number on the card is reversed, it remains a three-digit number. The tens digit of these numbers can be $1, 2, 6, 8$, 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). The tens digit of these numbers can only be $0, 1, 8$, 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 $\frac{1}{2}(80-12)=34$.