We've updated the examples in the
system requirements article to reflect 4.0 which hopefully give you an idea of processing power required for different scenarios. Actual requirements can vary with different cameras, scenes, and activity levels, but these should be representative a typical home installation.
Any of the NUCs, even the dual core i3/i5 variants, should be able to get you 8+ hd cameras. The quad core i7 even more, though for 10+ you'll want to make sure you have at least 16 gigs of RAM and a speedy storage drive. There will be a number of places to buy NUC+cameras+SV bundles this year, they're a nice little form factor if you're looking for a machine to tuck away in a corner or attach to the bottom of a desk.