Hi vne147,
I simulated the circuit using five LEDs in group at a supply voltage of 21V. There is no way to control the transistors with a maximum base voltage (theoretically) of 12 to conduct 21V. (The maximum I got out was 11.6V)
Reducing the number of LEDs to three the group can be well controlled. I used base resistors of 6K8 for each transistor and a pulldown resistor on each counter output of 10K. In practical design the pulldown resistors might be a 10 element SIL-package.
Not using the pulldown resistors the circuit becomes unstable and produces error messages while simulating. With the pulldown I "played" the circuit at least 50 times with no error message, doing what the counter outputs dictated.
Using three LEDs in a group the OP might use just one power supply which saves cost.
It also increases the number of flashes.
Two counter circuits won't suffice, but adding a third one shouldn't be a problem, increasing the number of LEDs to 501 total at a spacing of 1.996cm instead of 2cm
BTW, I suggest to connect the MR pins (15) of the counters to ground as well. It won't hurt.
Attached are four screenshots of different groups being lit. None is glowing dimly.
I put the current limiting resistors of 27Ω at the high side. The resulting current for LEDs Vf=3.5V, If=20mA is 18.6mA.
Regards
Boncuk
P.S. The pulldown resistors at U2 weren't changed to 10K, but at 1K they did well.