Hy alok,
Presumably, you are concerned about heat dissipation, but without knowing the type of LEDs you are planning on using, or the current level you will be driving them at, an answer to your question would be difficult, but as a rough stab I would think that a minimum spacing of the radius of the led would be adequate.
In general though, the important thing is to keep the LED junction temperature below the data sheet absolute maximum. To do this you need to lose heat from the LED by conduction and convection.
Conduction relies on contact with a heat sink which can comprise the traces on a PCB. It follows, therefore that the traces should be made as thick and wide as possible.
Convection relies on the free flow of air over a hot component, bearing in mind that convection currents flow upwards as hot air is lighter than cold air.
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