kyear2
International Coach
Only reason I use 5 is because that's the number of traditional test series, if you want to use 4, then fine as well.Any number is arbitrary. 5 is a lot more valuable than 1, but not too different to 4.
I think 10 or 15 is probably a bit low, so I use groups of countries. I prefer records that are good against every unique country, but there is just too much variance.
I'm saying that's what I use for consistency, and even then, not imposing it for others.