Hi,
You will need more combs and cutters if you are on sandy ground, Greyface Dartmoor wool actually blunts combs quite fast.
For 20 sheep I would think you will need 3 combs and 6 cutters at the most, it always pays to have too many.
One thing I will say is on my shearing machine I have a safety clutch for stopping the handpiece if barbed wire or something blocks the cutter, Greyface Dartmoor wool is the only thing that has caused it to slip.
Hope this helps,
Thanks,
Philip