Each 7135 chip uses ~ 200 micro amps to turn on and the Attiny chip can supply ~ 40 mA per I/O pin. 40 mA = 40000 micro amps or 200 - 7135 chips or 25 - 8 x 7135 boards. I doubt that 24 slaves from 1 master would work but how many have you done or heard of?
The ATTiny chips struggle a bit to output 40mA. A more reasonable number is about 20mA, especially if you’re using PWM to turn the 7135’s on and off rapidly…
There is no reason why a single Master couldn’t drive at least 10 Slave boards.
With custom firmware (E1320 or DrJones), slaves could be hooked to multiple ATTiny pins which would allow that number to multiply. A simple and cheap P-Channel MOSFET could also be used to increase drive current to a few hundred mA which would allow thousands of 7135 chips to be controlled from a single ATTiny Pin.
PPtk
Thanks, I’m working on a 7x XML old school dive light mod(acrylic face plate is 110mm) and haven’t decided whether to use a Makita BL1830 with a Taskled Hbflex or a master/slave mod with several paralleled 18650’s. Your description reminds me of a pilot show about a paraplegic ee who makes a sort exoskeletal nano-suit(not Iron Man but IC Man).
Is there a reason for choosing P-channel for this?
You need a high side switch to turn on the AMC7135s’ VDD. Their ground contact is in the high current path.
Yes, there is.
You have to control the high-side (VCC, not ground) in order to turn on/off a 7135 Chip. P-channels mosfets are made to do just that. With an N-Channel, you have to take the Gate above the Source by a few volts (Vgs) in order to turn it on. Works great when switching the ground path, but not so well when you’re switching the high side. P-Channel on the other hand requires that the gate be taken Below the source, meaning that we can just pull the gate to ground to turn it on and take it to VCC to turn it off.
Golden. Great explanation. Thanks, both Dr and PPTK!
I only have n-channel FET, I would've used that for future project and scratch me head why it flopped.
Time to ebay some more. 
Sorry to hijack this thread but would it be possible to connect several of these boards together to produce 12 or more amps ?
That’s what the master/slave mod does. It uses the processor of one board to control the 7135 chips of the other boards. It can be used to supply the 9 A needed for an SST 90 with the output of all 3 boards supplying one led or the outputs can be kept separate to supply each led with its own dedicated current source. Allows one master to control boards with different #’s of 7135’s to different types of LEDs(3A board to XML, 1.5A board to XPG, color mixing, etc. ). See this thread for the best “how to” and this thread for info and links to other useful stuff. Good luck
In the last few weeks this has become my favorite circuit. No need for fancy expensive drivers just keep stacking chips/boards.
What do you guy think about about a hybrid circuit of the master/slave system in combination with the Poormans Multi-lux and trying to power 3 sst-90’s from 3 li-ion using 3 2800ma boards with a few extra chips.
If you’re up for a long read, check out this thread about my variation on the poor mans multilux. Might contain some useful bits. The biggest problem you run into is that the voltage difference between charged and uncharged series batteries is more than the chips alone can handle when using more than 3s with modes and adding a dropping resistor reduces in spec run time unless you also add an additional switch to cut out the high wattage resistor partway through the discharge cycle. That said, I’m happy with my results so by all means, carry on and let us know how it goes.