Drive multiple LEDs using transistors?

Alright so I feel like making one of those fountains that uses a stroboscope to freeze the water droplets, I'm going to buy stripboard and make an LED arraw, however to drive all the LEDs I need to use transistors as relays, but I'm really not sure how to do it. I have a bunch if NPN transistors, and I'm thinking of having an array of 20 LEDs, which will need more than 5 volts to power it. How exactly do I wire this to an arduino to turn on the array?

You can have the LEDs in parallel, each with its own current-limiting resistor, then switch the lot with the transistor.

You can put them all in series and use a higher voltage supply - one current limiting resistor is all that's needed.

You can have several "strings" of series LEDs (plus resistor) and have the strings in parallel.

So long as the transistor has adequate voltage, current and power ratings only one transistor is needed (assuming
it needs no more than 25 to 30mA base current - that's the safe limit for current from an Arduino pin.

With 5V source, you will need the LEDs arranged in parallel 5V to Anode, cathode to resistor, resistor to collector, emitter to Gnd,

+5--|>|---///-- C--E--Gnd.

Arduino drives base thru a 220 ohm resistor.
Wire as many LED-resistor strings in parallel as need.

Resistor LED value = (5V - 0.7V - (Vf of LED))/.02A
5V is the source. 0.7 is the drop across the resistor (may be lower, down to 0.3V). Vf is the voltage drop across the LED. For white LED, could be up around 3.2V.
.02 is 20mA, most LEDs have that as max continuous current.
Adjust as needed for the parts you select.
If you have a higher voltage source, you can wire the LEDs in strips, maybe 4 or 5 in series, and reduce the current needed accordingly.
For example, 5 strings of 4 LEDs, 20mA/string, only need 12V, 100mA source,
vs 20 LEDs at 20mA each which needs 5V, 400mA source.

Most NPN transistor bases look like a diode to Gnd, with 0.7V drop.
So with 5V out of the arduino pin and say 20mA to saturate the transistor (turn it full on):
(5-.7)/.02 = 215, 220 is a standard value.
Adjust as needed for your parts.
Arduino guarantees 4.3V out for a high with 20mA load, so measure & adjust as needed.