You can approach this by adding another dimension when creating the equivalence classes. Consider 3 possibilities of the field in question:
N = 0
= no product in stockN = 1
= only one product in stockN > 1
= several products in stock
Writing tests for (1) and (2) is trivial, you already know how to get the boundaries.
For covering (3), you can pick any N > 1
and identify the boundaries for it.
The actual value chosen for N
(when N > 1
) is not important (unless you want to cover field overflow - e.g. integer overflow), so you will simply control it on your preparation steps and/or test data.