This very thing happened in one of my recent games (and if you are one of the players in that game, stop here...).. the party cleric was killed first, and after that, well things went downhill. The mage with a 10 str was next to go, and they got the only fighter down to a 1 str before he fled away on horseback..
That left several very evil shadows in a village with a bunch of villagers that could not harm them..
The players have no idea how many shadows were created from this, but needless to say, the survivors want the players to cleanse the village and put their slain to rest.
I ruled it something like this:
A given shadow can create as many subshadows as he has hit points.. so if he has 17 hp's.. he can spawn up to 17 shadows.
A sub shadow can create spawn, but they count towards the maximum of the sponsor shadow until a number of days have passed equal to the sponsor shadows hp.. (i.e. 17).. then they all become full shadows. keeps 'em from overrunning the world.
another idea I came up with:
A shadow cant spawn in broad daylight.. gives them a reason to attack from darkness..