rule of thumb is one judge per team . . . plus table captains and other volunteers.
KCBS style competitions can be held anywhere. If you're talking a sanctioned event, I believe there are rules (guidelines) regarding the facilities but I would imagine KCBS would be happy to sanction an event in a backyard.
That would have to be one huge backyard to accommodate 20 teams . . .
And lastly, if you're talking a sanctioned event, I'd say to go straight to the source and ask them.
Good luck with it!