If a Nabassu (Mordenkainen's Tome of Foes) were to use its Devour Soul feature:
A nabassu can eat the soul of a creature it has killed within the last hour, provided that creature is neither a construct nor an undead. The devouring requires the nabassu to be within 5 feet of the corpse for at least 10 minutes, after which it gains a number of Hit Dice (d8s) equal to half the creature's number of Hit Dice. Roll those dice, and increase the nabassu's hit points by the numbers rolled. For every 4 Hit Dice the nabassu gains in this way, its attacks deal an extra 3 (1d6) damage on a hit. The nabassu retains these benefits for 6 days. A creature devoured by a nabassu can be restored to life only by a wish spell.
Can it only increase it's hit dice once from the strongest creature it has devoured or does the effect trigger repeatedly for multiple creatures? (i.e. does it stack for each 2+ hit dice creature it uses this feature on)
From the description of the creatures ability states that it grants actual an hit dice increase, and there is no rule restricting accumulation of these. Since the hit dice are temporary, it can only stack so much at any given time. If these were temporary hit points, instead of actual hit dice, we would know that it does not stack, due to the Temporary Hit Points rule, but they aren't, so that doesn't apply.