Basically if you set clustersize=6 and numclusters=1 for diamond ore, that means that Thermal Expansion will generate on average one extra vein of diamond ore, in addition to what vanilla generates, and that extra vein will have a maximum size of 6 ore blocks.
One extra vein per what? We don't know for sure. TE hooks into the vanilla ore generation mechanics and just tells it to "do one more vein with size 6". It's probably a percent chance per chunk mechanism, similar to how GregTech generates iridium ore. Maybe something like 25% chance for each chunk to receive that extra vein. But that's pure speculation on my part.
P.S.: We get these "why are diamonds rare" threads every time a new modpack comes out - just try the forum search. And every time someone goes and scans a statistically useful area with an external tool and gets exactly the expected average amount of diamonds per chunk. There is no actual shortage, people just think there is because of:
- Clustering Illusion: diamond distribution is fairly random, and it's perfectly possible to have a bad luck streak, but people misinterpretate it
- Increased Consumption: because each new mod adds additional needs for diamonds, people have a harder time satisfying the demand, and therefore it feels like diamond is harder to come by when in fact they still find the same amount as always
- Richie Rich Syndrome: Coming out of a well established world where quarries and turtles and tunnel bores funnel a neverending stream of diamonds into the storage chests without the player having to lift a finger, it feels twice as difficult when coming into a new world where they have to dig out every diamond by hand.