6. There are 900 three-digit numbers $(100,101, \cdots, 999)$. These three-digit numbers are printed on cards, with ten three-digit numbers on each card. Some cards, when flipped, still show three-digit numbers, 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 I $E$. Therefore, some cards can be used in two ways. Thus, at most, _ fewer cards can be printed.
6. If the number on the card is reversed and still a three-digit number, the tens digit can be $0,1,6,8,9$, while the hundreds and units digits can only be $1,6,8,9$. There are $5 \times 4^{2}=80$ such three-digit numbers, but some of them, when reversed, are still three-digit numbers and are the same as the original 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, corresponding to $1,9,8,6$, for a total of $3 \times 4=12$ numbers.
Therefore, the maximum number of cards that can be saved is $\frac{1}{2}(80-12)=34$ cards.