I'm going to make few parts of plane cockpit. I need to use about 20 encoders, about 10 character LCDs and a lot of switches/triggers/buttons.
I think it's not a problem with LCDs and buttons, because I can use shift-registers, but I don't know what should I do with encoders (PWM signal).
I'll appreciate for any suggestions about how to use a lot of PWM inputs in one board.
It depends on the signal speed and the resolution you want. But you can sample this inputs also using SPI or GPIOs. You can them regularly using a timer and then calculate the pulse width or the encoder steps in software.
For quadrature encoders there are chips you can read out over SPI or bus interface.
Another possibility would be to use an FPGA or CPLD to preprocess the inputs.
If it's safety relevant (plane cockpit), you should take care about hardware and software architecture/design, but you know that probably.