2-4 would be okay, 4 is on the high side, 2 on the low side.
2 would run a 75 gal nicely, 4 and CO2 would be nice also.
Depends on plant choice also.

also, you will need to space the lights way apart to get decent coverage in the tank, say 16-18" apart with two, whereas 4-5" spacing works well with 4.
Also, run the 2x54 for 10 hours and the 4x54 for 2 hours etc.

You have some options.
On decent sized non CO2 tanks, and low light CO2 tanks, T5's are ideal.

Regards,
Tom Barr