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I'm interested in building a monitoring system for my 4-zone HVAC system, which will require monitoring between 8 and 12 separate 24VAC circuits. Following a suggestion in this question, I plan to use MID400 optocouplers to convert the 24VAC signals to digital, but I'm not sure how to get the 12 separate inputs into the PI.

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You could look at the MCP23017 i2c 16 input/output port expander.

It gives you 16 i/o ports and communicates to the Raspberry Pi via I2C so it only takes 2 pins.

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