The Attiny85 has 8 I/O pins:
- reset
- Grnd
- VCC
- 5 I/O pins
I had 2 different code baselines: Narsil (2 chans) and Narsil Triple (3 chans). It's now recently has been combined into one code base I'm calling "NarsilM" for multi-channel support. These two versions were diverging and getting difficult to keep both up to date.
Your hardware can be specified at compile time.
For an SRK, yes, we can do 3 channels, a switch and a indicator LED (battery voltage monitoring can be done internally). But for 2S uses (like XHP50's or XHP70's), we need external battery voltage monitoring, so need another pin - an "LDO" must be used to reduce parasitic drain. Right now, the indicator LED is sacrificed.
So NarsilM can support it all, accept for the combo of 3 channels, LDO, and an indicator LED. Bad news is for 3 channels, it's very close to the limit of program space. I haven't released NarsilM yet because of needing further testing of all the configurations, and it would help to do some optimizing to reduce the code size.
Back a year ago, yes - 2 channel was the way to go. Not sure, but think 3 channels was just coming out, maybe not fully supported yet. We committed early to the electronics design though.