You really need to determine exactly what you want before making any decisions.

When considering a double/triple shift operation the simple answer is one battery per truck will not suffice. To identify the most efficent truck you need to get the KW/h rating for each truck and establish the highest capacity battery available for each machine i.e. the Ah rating of the battery. With these figures you can calculate the theoretical maximum hours useage for each machine. However, you also need to consider the application for each machine. For example a truck which uses the lift hydraulics more per hour than another machine will draw more power from the battery than a machine that is just being driven.
If you want to minimise running cost and eliminate the need for battery changing then you should investigate fuel cells. The intial capital cost will be higher as will the implementation.