I own Raspberry Pi 2 B and I wanted to create something like Novation's Launchpad. How many buttons can I connect through GPIO at once?
Without add-ons and using the Pi's gpios 26 (or 28 if you are happy to forgo HAT compatibility).
If you buy something like a MCP23017 (port expander) you can have 100s. They provide 16 additional gpios each and 8 may be connected to an I2C bus (the I2C bus is Pi pins 3 and 5).