I am just installing, I want to install in a simple hallway 18' long, 3-4 lights. I just wanted to see how many and what size 4",5" or 6". 3 or 4 lights. There is only one mini chandalier.

I'd go with an R30 (5'/6")sized can and use three, especially considering the chandelier.

R20 (4") sized can will work too but the light spread will be a bit narrower, meaning the walls will not be lit as well.

As far as wiring them goes forget the terms series and parallel. This really does not apply in AC building wiring. It is all technically parallel.