With your dragon fruits seeds ready to plant now, you are ready for starting the dragon fruit seeds growing. You can either sprinkle the dragonfruit seeds sparsely (a few mm apart is all you need), or you can individually place them. If you place them or sprinkle them, keep the dragonfruit seeds 2-4 mm apart and you will have plenty of space between them as long as you harvest the dragonfruit seedlings early enough that the root masses are still moderate (and hence not entangled extensively with one another).
After you have sown the dragonfruit seeds on the perlite or tezontle top layer, it is best to sprinkle a final and even thinner layer of perlite or tezontle. You just need to cover the dragonfruit seeds. This will help keep the top seed layer humid. You don't need to do any more than cover the seeds, the seedlings will easily push their way up through the covering. If you use more, it should not be a problem, but it is not required - the dragonfruit seedlings will just push up through a thicker covering of perlite/tezontle.
You want to keep the dragon fruit seeds slightly separated from the soil medium mixture to keep them from becoming too wet (as the soil mixture, while well drained, is still going to be more damp than the over-layer of perlite in which you placed the seeds).