I've been in ten syndicates over 23 odd years (at any one time, usually a member of 2-3: it works for me), from 2 to 20. I don't think that there is an ideal size - to a fair extent I prefer larger groups. You'll find in a group of 20, that only 6 fly the aeroplane regularly and, mostly, this works fine.
I'm presently in a group of 11 and another of 17. Neither do I fail to get the aeroplane when I want it with any regularity - both are very well run and well looked after aeroplanes: and whilst I try to pull my weight, I also (having had the odd prior bad experience) have become rather more cautious of who I'm joining! So I tend to disagree with those saying that only small groups work.
G