I collected all the sheep I saw, brought them to the pen and only 8 were there. 10 are needed for the mission. I counted them to be sure and there were only 8 there. I have looked everywhere for the other 2 but I can't find them anywhere. Also it seems they have a barrier that sends them back to their spawn point once they've reached a certain distance from their spawn. (I'm not sure which version I have. I selected the more likely of the 2.)