What board do I use?

Hi!

I'm new here, in the forum and in Arduino's world. I want my first project to have 25 LEDs and I don't know which is the best board to use. I read a lot of posts in forums and searched in Google, but I haven't found an actual answer.

Thank you so much in advance!

An UNO can handle 20 LEDs (@ low current) directly. Usually you will use driver circuits / shift registers to handle 'any number' of leds. In this case any arduinu can do the job. Take a look at the RGB-led 2812B. With theese you can easely handle hundreds of them

There is no actual answer. Addressable leds (see above) is one approach, shift registers or port expanders (I2C/SPI) are another when your short on pins. Else your looking at e.g. a Mega where you can directly connect them. You need to take into account how much current the microcontroller (or shiftregister / port expander) can sink or source; for the micro on an Uno or Mega its 20mA per pin (safe limit), 100mA per port (8 pins) and 200mA total.

The approach also depends on if you need to be able to dim them or not; I don't think there is an Arduino board with 25 PWM outputs.

You can connect 20 ordinary LEDs with 5 pins and switch each on and off. You will need 5 series resistors. But only one led can be on at a time. If you need more than one led on at a time, you can still use 5 pins but add 5 npn transistors. If that sounds interesting, ask for more details.