Personally, I think the YBR pattern looks better with more variety, not less. I've made that pattern and didn't have enough assortment and, yes, some of the same colors touched. Did not like the result. Since the size you're making calls for 24 FQ's, you can probably get away with some repetition but my suggestion is to keep it to a minimum.
Yes, an odd number of fabrics is the way to go, i.e., 21 different fabrics and 3 repeats. It doesn't make necessarily make it easier but it will make your quilt look much better.
Hope that helps!
Jane