How would I wire 8 load cells into a single system?

You would use multiple summing junctions boxes in series to handle more than four load points. The number of load cells you are able to connect to a single instrument would depend on the specification of the particular instrument.

The summing cards should be positioned as close as possible and use certified C2 cable runs between the summing cards and the weight controller.

I have attached a diagram to help explain the concept.

One condition is you cannot use the Integrated Technician summing junction card for more than 4 load points.