I am planning on using the PiTFT - 2.8" TFT 320 x 240 CapacitiveTouchscreen with my Raspberry PI 2 Model B with 40 GPIO pins and would like to know how many of the 26 GPIO pins it uses even though it completely covers all 26 pins.
From your link.
Uses the hardware I2C Pins (SDA & SCL), SPI pins (SCK, MOSI, MISO, CE0) as well as GPIO #25 and #24. All other GPIO are unused and you can still share the I2C pins with sensors, LED drivers, etc. Since we had a tiny bit of space, there's 4 spots for optional slim tactile switches wired to four GPIOs, that you can use if you want to make a basic user interface. For example, you can use one as a power on/off button. See below for the link to get the optional tact switches, they're not included.
Therefore it uses 8 GPIO.
There are 40 pins on the Pi2's expansion header, of which 26 are usable GPIO. That means you have 18 spare GPIO.