Avoid connecting LEDs directly in parallel!!!
The issue arises because LEDs, and diodes in general, exhibit slight variations in their properties. Each LED has a different forward voltage curve, leading to some LEDs receiving excessive current. This can result in damage to those LEDs. If one LED fails, the current will then pass through the remaining LEDs, causing a chain reaction of failures until all LEDs are non-functional.
How to mitigate this issue?
The best solution is to connect all LEDs in series using a single current-limiting resistor (or a current-limiting circuit). Aligning LEDs in series reduces the voltage across the resistor, minimizing energy losses. Your power supply voltage must be sufficiently high for this arrangement. If it falls short, you can create multiple series of LEDs, each with its own resistor.
Ensure that the voltage across the resistor is sufficient to prevent significant variations in current. This can occur because ofvariations in supply voltage or the variations in LED properties.
The best thing to do is to keep a safety margin in mind. Let's say the max current of the LED's is 20mA, keep the current at 15mA or so. This is also better for the lifespan of the LED's
A circuit
To complete my answer, a little schematic. I am assuming your LEDs are max 20mA and have a forward voltage of about 1.8V. Using this schematic, the current through the LEDs is a little under 14mA.
