8, 8, I forget what is for
Showing up at a large human-created food supply like a dump is not how these entries are presented or how I suspect they are normally used. Six black bears at a dump or six grizzlies at a river during spawning season is a planned encounter; 1-3 black bears or 1-6 brown bears is a random wilderness encounter, and IMO encountering anything more than 1 bear (other than cubs, which are generally noncombatants) should be pretty unusual.Black bears more or less do so in real life. In north-central Ontario they'll show up to town dumps en masse to eat the garbage. So long as food's abundant, they seem quite sociable with one another.
Even where bears are found together, they aren't exactly social. It's not like they will attack as a group; if you encroach on salmon feeding grounds, I think you might get in trouble with one bear if you threaten its fishing spot, but it's not like they are going to mob you.
Unlike other animal entries, there are no stats for females or cubs, so it clearly wasn't intended for the sort of situation where a sow appears with 1-2 cubs. So the setting implied by the MM and Schedule C suggests that grizzlies are roaming the land in packs.