Well...usually it is easier to search for an even number party because of a quirk in the Disney reservation system that wants to fill every seat so if you input an odd number party such as 5 it bypasses tables of 6 because it wants to fill the table with 6 guests. If you input 6, there's a better chance of finding a table.
However...with CRT and any other prepaid reservation we have found that the system is more likely to find tables for the exact number possibly because it requires full pre-payment for the entire party. We've never had problems finding a table for 3 at CRT for our family, but at many other restaurants we've had better luck looking for a party of 4.
If you book for 6 you will eventually have to call to change the reservation to the correct number of diners and arrange a refund to your credit card if paying OOP. Just be sure the CM understands you just want to modify the reservation and not cancel it.