The information you are looking for can be found in the ATMega32u4 datasheet. The footnote for Table 29.1 explains the groups of pins and the total current each group can sink or source. A Teensy 2.0 has 21 I/O pins, and 19 of those will be used for the keyboard (I am assuming you are building a 60%). If the remaining two pins are used to drive all the LEDs, the maximum current you can sink or source is 2 * 40mA (absolute maximum) = 80mA.
If you arrange the LEDs like this:
(I/O pin A --- Resistor --- LED --- LED --- Ground) x 15, and (I/O pin B --- Resistor --- LED --- LED --- Ground) x 16
then you can get 2.5mA per LED. This is about 1/10th of the nominal maximum LED current, so they will be very dim.
I would recommend you use a transistor or MOSFET controlled by the teensy to provide power to the LEDs instead of powering from the Teensy.