ARS will cycle through the trunks in the pool, in a circular fashion.
You could put the trunks in different pools, and add the pools as alternate routes in the ARS.
For instance, lines 6 through 10 are in pool 70, and 1 through 5 are in pool 890. ARS route 1 is 70, while route 2 is 890. It will use ANY ONE of lines 6 through 10, but if they are all in use it will use ANY ONE of lines 1 through 5.
Remember that 911 uses pool 70 only, unless you build a specific route for it.