Check 74LS248 datasheet first.

74LS248 uses 4bits of MCU to drive one display (without dot).

Thus, for example, by using only PORTB, you can drive two displays with no scan frequency.

Also, if you use enable/disable thingie of these IC's, you can reduce the number of pins required for driving all 18 displays. But, this time scan frequency will be on table again. You should experiment.

For example,

You can drive 4 displays via PORTB and PORTC without any scan frequency.
IF you also use enable/disable pins of these 74LS248 ICs, then you can drive say 8 displays.
This way, you will not get flickering because you are going back and forth between two displays on each port.

May be, your experiment will give you an acceptable light level if you drive 3 displays via one port using enable/disable feature.

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