Like said above, depends on load, but as a general guideline I've got 9 cameras running just fine on an ebay office desktop (an Optiplex or similar small form factor [SFF]) I got used off ebay. In the US, you can get something i5-6500/8GB/128GB SDD for $250-300 shipped. This works fine for me, and I have room to go up to easily 12 cameras or more (these are 2-4 Mpixel cameras). Storage is typically done by adding ~4TB WD Purple drives. the SFF office computers can fit a SDD and two hard drives in the box, typically.
Seriously, until I read the
wiki on picking a computer for
Blue Iris, I had no idea about these refurbs/off-lease SFF desktops. Just go to ebay, put in i5-6500, choose "Desktops" or similar what comes up to avoid getting listings for parts, and then sort by price. Avoid anything that is 6500U or 6500T as those are lower power versions, less processing oomph.