All the big 3D Printer suppliers always show off braingears printed as single items. This is to show that even our humble hobby printers are capable of printing a brain gear - all be it in seperate parts for assembly later.
You might need to scale the parts up by 200%. You could even try and scale this thing up as high as your printer will allow you to.
You'll need to orient the parts so the flat sections sit on the base/build plate of the printer - that's how they were designed.
Print all the parts needed, then assemble. Glue all the links together at ounce before sliding them in - don't try to glue each link individually.