Azhar a slack box is not practical for that many threads as each thread needs its own setup. more information would help in determining how to solve this.
is each thread supplied from a separate spool if so can you use spool rotation as a signal? How many broken threads are allowed? How large is the budget for this?How much time to complete this?

The slack boxes that I useed were on photographic processing equiptment and had to be very carefully done to avoid damaging the product. they also needed to trip as soon as the line stopped when a line broke to minimize over processing damage.