Yes it’s the odd number, when using a boost driver the 3V LEDs are put in series or a combination of series/ parallel, for example 4 series (4S = 12V) or 2 series 2 parallel (2S2P = 6V).
Odd numbers work too like 3S = 9V, but higher is limited by the maximum output voltage of the boost converter used, in the case of the comonly used MP3431 it’s 16V, 5S with 519As would actually work, since they have a low Vf of ~3.1V at 2.5A so 5x3.1= 15.5V, but other LEDs with higher Vf wouldn’t, and 7S would not as well, additionally, higher output voltages are less efficient, see The measurements I’ve done of my 6V and 12V drivers for example (2 last tables).
Putting different amount of LEDs in parallel like 2S+2S+3S or 3S+4S would result in pretty bad current disbalance between the LED strings.
It would be much better to simply remove the center LED and do 2S3P.
For single input cell flashlight boost is better than buck since it can have full output regardless of the cell’s voltage voltage whereas with a buck, once the cell’s voltage is close to the LED’s Vf the output starts to drop (same with Linear but can be closer because lower driver resistance), the efficiency and output of available boost converter is also higher. The advantage of buck is that like it is done in the Fireflies buck+FET driver it can be combined with a direct drive FET channel for more output (than boost) when the cell is full.
I was talking about low~high mode and sustained (thermally limited) current where using 7135 chips are more efficient than direct drive. Both custum linear driver and 7135 linear driver can be high current.