Actually, just checked in-game:
High Caps:
Needler - 192
Laser - 192
MLRS - 192
Missile - 192
Flak - 96
Lightning - 96
HBC - 12
Sniper - 192
Counter Sniper - 19
Counter Dark Matter - 19
Counter Missile - 19
Tractor - 192
Grav - 58
Tachyon Beam Emitter - 19
Ultra Low Caps:
Needler - 24
Laser - 24
MLRS - 24
Missile - 24
Flak - 12
Lightning - 12
HBC - 12
Sniper - 24
Counter Sniper - 19
Counter Dark Matter - 19
Counter Missile - 19
Tractor - 24
Grav - 58
Tachyon Beam Emitter - 19
Stuff like the counters, gravs, and tachyons are useful mainly via coverage, not by comparison to unit count, so they don't scale. If they don't scale, the multiple-of-8 thing is irrelevant.
So which turrets aren't multiples of 8 that need to be?