The quests are designed for 4 players, as I understand it. CR 2 means 4 level 2 characters. If this is correct then why not allow for someone soloing to purchase and use 3 hirelings if they want to, instead of just one? This way they can have their 4 level 2 characters for the CR 2 quest, for example. If you have a full party of players, just cap it the same as if you had only players in the party.