16 souls is generally enough to get some useful equipment; with only 8, even if you score one of the good early game weapons you can't use it, and you have to struggle against a level 1 encounter with your terrible starting gear.
And some of the lvl 1 encounters are really hard. You need some really good luck rolls against silver knights with starting gear for example.
I recently started a campaign play through with assassin and knight where the 1st and only encounter was silver knight, hollow soldier and hollow crossbowman. Luckily due to the room layout my herald could kill the crossbowman, tank the damage and let the assassin "crit" the silver knight.
It all depends on how it gets together. I have just found out I am very unlucky with my dice throws, regularly throwing 3 blank blacks or 3 blank greens.
I think the starting 16 souls is about right, but makes 2 player games pretty hard as you don't get any bonuses at all.