I think it would depend on a few factors, namely the weather and the location of the herd. It would be easier to navigate a city in a small herd while a larger herd could survive better in a park, forest, or countryside. Summer months will bring more food, good weather and more fluffies while winter and fall would kill off the vegetation and the cold alone could kill a large number of fluffies.
And there is a difference between a family group and a herd. Family groups led by a mated pair or a single mother/single father, along with any number of foals of different ages. A herd led by a single Smarty (either an arrogant hellish tool who rules through fear and aggression or on occasion a dependable, intelligent (for a fluffy), and ambitious Smarty Friend. Fluffies in a Smarty led herd are more likely to die because of the regime of their leader or leave because they don’t like how the Smarty is leading.
While family groups are small between 2-4 adults with any number of young foals. Herds themselves are larger, could be a Smarty and his/her chosen special friend. An appointed head toughie and toughies. Then are pregnant mares and/or their mates. I also like to think that fluffies are both monogamous and polyamorous, occasionally bonding as Special Friends or a stallion might have multiple mares and mares might have sex with multiple stallions without forming emotional connections- it depends on the fluffy.
After that in the hierarchy would be nummie finders, this could overlap with the pregnant mares/mates, and after that are poopie fluffies/nest attendents. Depends on the herd those fluffies could be relegated as the worst of the worst in charge of taking the brunt of the herd’s scraps and shit. They also could be fluffies in charge of cleaning up the nest and making it habitable for the herd itself.
So in a city scape a herd could be (1) Smarty Leader, (2-3) toughies, (0-3) pregnant mares, (1-2) nummie finders, (1-2) poopie fluffies/nest attendants. Evening out at between 5-11 adult fluffies and between 4-6 foals per mother on average, though it could be more or less depending on nutrition, the age of the mare, stress, illness, etc…
In the countryside a herd could be (1) Smarty Leader, (2-7) toughies, (12-25) pregnant mares, (5-9) nummie finders, (5-8) poopie fluffes/nest attendants. Evening out at between 25-50 adult fluffies and same difference for the foals.