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 861. Therefore, at most, $\qquad$ fewer cards can be printed.
6. 34 The following factors should be considered: (1) Which digits have an "inverted" effect? (2) Some digits, though they can be "inverted," remain the same after inversion, such as $0, 1$. (3) As a three-digit number, 0 cannot be the leading digit, so if the original number ends with 0, its inversion is not a three-digit number.

The cards that can be used in both directions can only consist of the digits $0, 1, 6, 8, 9$, and the hundreds and units digits cannot be 0. Therefore, there are $4^{2} \times 5=80$ such three-digit numbers. However, this includes three-digit numbers that remain the same after inversion, where the tens digit is $0, 1$, or 8, and the hundreds and units digits can be $(1,1), (8,8), (6,9)$, or $(9,6)$, totaling $3 \times 4=12$.

Thus, the maximum number of cards that can be saved is $\frac{1}{2}(80-12)=34$.