Maximum pins that can be used for i/o in RaspberryPi2 J8 header?

According to the RasPi Hardware Wiki http://elinux.org/RPi_Low-level_peripherals#Model_A.2B.2C_B.2B_and_B2, there are 28 GPIO pins on the RasPi Model A+, B+, and 2.  I think you can configure all of them as GPIOs, or use alternate functions like SPI and I2C.  However, I haven't tried all the pins myself so YMMV.

It doesn't look there there are any reserved pins any more -- they've all become GPIOs.

RasPi GPIOs use 3.3V logic and are not safe for 5V signals.  There are other useful warnings at the Wiki.

Update:  According to the RasPi Model B+ "reduced" schematic https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/Raspberry-Pi-B-Plus-V1.2-Schematics.pdf, there are only 26 GPIOs.  GPIOs 0 and 1 are reserved for ID_SD and ID_SC, an I2C bus for talking to a HAT's EEPROM.  However, also according to that schematic RasPi only checks that I2C bus at boot time, so you might be able to switch them to GPIOs later.  However, whatever you attach must expect I2C signals at boot time and must not confuse the OS by responding to those signals.

Some of the other signals may have default behavior that could confuse your add-on board.  For example, it may be that RXD0 and TXD0 look like a GNU/Linux terminal.  Also, some of the signals (such as I2C) may have pull-up resistors.  I think there's also a GPIO pin that used to select "safe mode".  You'll have to be careful with that one as well.

There may be useful discussion of this at the official RasPi forum https://www.raspberrypi.org/forums/.

According to the RasPi Hardware Wiki http://elinux.org/RPi_Low-level_peripherals#Model_A.2B.2C_B.2B_and_B2, there are 28 GPIO pins on the RasPi Model A+, B+, and 2.  I think you can configure all of them as GPIOs, or use alternate functions like SPI and I2C.  However, I haven't tried all the pins myself so YMMV.

It doesn't look there there are any reserved pins any more -- they've all become GPIOs.

RasPi GPIOs use 3.3V logic and are not safe for 5V signals.  There are other useful warnings at the Wiki.

Update:  According to the RasPi Model B+ "reduced" schematic https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/Raspberry-Pi-B-Plus-V1.2-Schematics.pdf, there are only 26 GPIOs.  GPIOs 0 and 1 are reserved for ID_SD and ID_SC, an I2C bus for talking to a HAT's EEPROM.  However, also according to that schematic RasPi only checks that I2C bus at boot time, so you might be able to switch them to GPIOs later.  However, whatever you attach must expect I2C signals at boot time and must not confuse the OS by responding to those signals.

Some of the other signals may have default behavior that could confuse your add-on board.  For example, it may be that RXD0 and TXD0 look like a GNU/Linux terminal.  Also, some of the signals (such as I2C) may have pull-up resistors.  I think there's also a GPIO pin that used to select "safe mode".  You'll have to be careful with that one as well.

There may be useful discussion of this at the official RasPi forum https://www.raspberrypi.org/forums/.