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[Reply #15 on:]

"how many devices can be connected using ISP and a arduino as a Master ? "
Depends how you wire them up, doesn't it?

Use a part that buffers the signals as it passes them along, like WS2801, the number is probably reaaaaally high, limited by your abillity to distribute power.  That's basically what LED strips are - how long are those strings?

Use parts that are all driven in parallel, with SCK-MISO-MOSI going to all pins and unique chip select or latch pins, you are only limited by the number of slave select pins available.
For an Uno, that would be 17.

For some combination of the two, there are too many possibilities to count. As I described above, I demonstrated 20 shift registers controlled with just 3 pins, and made 5 sets doing of boards do that.

The schematic I showed had 64 parts being controlled - the key would be in proper distribution of the control signals and power.  Termination of the signals might be needed, altho with SPI at 1/4 of system clock, that's 4 clock cycles (250nSO) for signals to settle, making for a pretty tolerant system.