In theory, you can hookup hundreds or maybe even thousands, but you will need some additional components.
What those components should be depends on what you are controlling.
When you say "electrical devices" what is that? is it the lamps in you home, tiny little LED's, a motor or what?
The Intel Galileo board can only supply very small currents on the output pins, so if you want to control anything thats more powerful than an LED, you will need to use some sort of driver circuitry, and that needs to be designed to fit your load (the lamp, motor, etc.). Even bigger LED's will require a driver of some sorts.
/Thomas