So Hunts...

[ispano]

B ranks were meant to be soloed.

A ranks were meant for a full party or an alliance of parties.

S ranks were meant for all the players in a zone or more.

Really? No. Let's try this.

Rank B maximum HP has been raised from solo to light party level, Rank A maximum HP has been raised from light party to full party level.
Although the Rank S maximum HP level was already intended for multiple parties, we have further raised the necessary number to require even more parties.

That means A Ranks were meant for a light party, ie 4 players. And an S rank was meant for something around an alliance.