Network topology and fibre

Hello all,
I am looking at various networks and how network topologies relate to the fibre network i.e. estimating the type and quantity of fibre optic cable required. I've done up the following simple schematic which shows a central core server which in turn feeds a fibre ring loop of 6 switches to create redundancy. Each local switch would then provide communications links at each site i.e. IP CCTV etc. They are also daisy chained as you can see. The fibre is the black line whilst the blue lines are local LAN/CATV or whatever at each site.

So if I was to build this network would it be sufficient to have a single 4 core singlemode cable to create the loop i.e. Each connection between switches would only require 2 cores (tx and rx)?. 4 cores seems overly little considering such a network could be expanded in a similar fashion to add in multiple of switches.