You get a total at the start of the game based on how many models you have that contribute faith. Basically, for every Veteran Superior, Canoness, Palatine, or unit of Seraphim you bring along, you get a faith point. The Canoness gives you a bonus one on top of the one you get for bringing a faithful unit.
However many faith points you come to the table with is how many you've got to work with FOR THE GAME. They don't replenish at the start of each turn or anything. You do get a point back each time a Faithful unit dies, though-- that's the Martyrdom rule. For example, my 2000 point Sisters of Battle army has 9 faith points at the start of the game. When they're gone, they're gone.
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