I have a very cheap Chinese flashlight (Torch for Brits) that has 24 matched white LEDs in parallel and the internal resistance of the three series very cheap "Super heavy Duty" AAA cells limits the current.
An LED has a maximum allowed current and will soon burn out if your circuit exceeds that current. An ordinary 5mm diameter LED has a maximum current rating of 30mA or 40mA and its brightness is rated at 20mA.
For testing the forward voltage of an LED I use a 9V alkaline battery and connect a 330 ohm resistor in series with the LED.
A red LED is about 2V so the resistor has 9v - 2V= 7V across it and the current is 7V/330 ohms= 21mA. A white LED is about 3.2V so with the 330 ohm resistor the current is 17.6mA.
Using such a high supply voltage as 9V for a 2V or 3V LED wastes a lot of battery power. Then an efficient voltage converter circuit is used to replace my 330 ohms resistor.