Do you mean rather small LEDs (nominal 20mA)?
They are white I guess. If the supply is 12V, each branch can be made with 3 LEDs in series. So for 50 LEDs, the possible matrices are:
3 * 16 branches = 48 LEDs
or
3 * 17 branches = 51 LEDs
To be in the safe side, each branch will need a current limiting resistor Rs (in series).
Rs = (Vcc – 3*V_led) / I_led
To calculate Rs properly, your first test would be to decide on the current I_led (lower the current, longer the LED life) and get a measurement of the LED forward voltage V_led at this current I_led.
Let us assume I_led is about 20 mA, and you have a 12V supply, the series test resistor is:
R_test = (12 - 3.5)/20 = 425 Ohm => 470 Ohm (standard value)
Now your test circuit consists of:
12V supply , 470 Ohm , white LED (cathode to ground).
Measure the voltages V_rs (over the resistor) and V_led.
The actual I_led is now equal
I_led = Vrs / 0.47 (in mA)
If the brightness of the LED is ok then the measured voltage on the LED is V_led.
Let remember that
Rs = (Vcc – 3*V_led) / I_led
It is easy now to found Rs for each branch.
Sorry, you are right I forgot to put my glasses
If you still need me, just say hello
Kerim