I'll assume you are asking about puck shape LED lights. Those typically have a slightly focused output compared to other lamping so I'd say they need to be spaced a little closer than normal. But that is offset by the greater than normal distance from countertop to underside of upper cabinets; the greater the distance the beam travels, the larger the cone of light expands.
However, LED lights, which I do love, have a lower lumen output. Pucks generally put out 200 lumens which is the rough equivalent of a 25w incandescent. So you may need more of them to give you the cumulative lumens needed for task lighting.
Lighting layouts are tricky and always site specific. So with my lack of knowledge of you cabinet run and needs, take this with a grain of salt. I would space them 18" and antipicipate using more than initially planned.
Phew!!! After all that, I much prefer STRIP style LED lighting for under cabinet lighting. There operate on 12v or 24v transformers, can be dimmed with a electronic dimmer switch and their lumen output is based on a linear foot measurement of the strip. 24v systems are better as they put out around 240 lumens per linear foot. The main advantage of these is the even output of light across the space.
These are awesome when you're doing a whole kitchen because each short stretch can be daisy chained so they are all on one circuit.
If you're doing one small area, stay with pucks.
Hope this helps.