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Well, that's simple! Adding onto the group only really adds onto the food and vet bills (which can be a right pain in the...) -- it's not much harder to look after three ferrets than it is two, or four than it is three
Especially when you've got a second pair of hands - if you've got everything properly ferret-proofed, then it's just a matter of making sure everybody's tummy is sufficiently tickled during the hour or two of supervised play.
They call it 'ferret maths' . . .
1 ferret - can't be alone! He needs a friend.. 2 ferrets. But if one of them passes on, he'll be alone.. 3 ferrets! But now it's uneven... 4 ferrets! And oh, what's one more... 5 ferrets!