Turning on multiple leds with arduino using transistor
A transistor is basically a switch that can disconnect ground or 5V. There are transistors of all sorts, and some can switch truly huge loads. 50 LEDs is no big deal.

So, the question then is this: Can a single 2N3904 switch 50 LEDs?

To answer that, we'd need to know first what kind of LEDs. But let's assume you are using the usual nothing-special variety. A reasonable estimate of the maximum current of these is \$20mA\$. If you have 50, then the maximum current is \$20mA \cdot 50 = 1000mA \$. Looking at the datasheet I see the maximum collector current for 2N3904 is \$ 200 mA \$. So the answer is no: you can not switch 50 LEDs with one 2N3904.

I suppose you have several options:


use multiple 2N3904
use fewer LEDs
use the same LEDs, but drive them with less current
use a bigger transistor (TIP121 is very easy to find)
use some other switching device


Of these, I think reducing the LED current or using a larger transistor is probably the most likely solution. Other switching devices (like a relay) are probably more expensive and slower.