Does Decree of Discord return all order units below 5 might or any number of units with total might 5, meaning that when you hit total 5 might you can't return anymore units?

The latter — you return "any number" of enemy Order units whose total Might is 5 or less, so once the sum reaches 5, you can't add more.

The card is not "all Order units with Might below 5." When you play it you choose the targets (any number, including zero) and their combined current Might must be ≤ 5 at finalization. So you could return five 1-Might units, one 5-Might unit, a 3 + a 2, etc. — but you can't keep adding once the total would exceed 5 (e.g., 3 + 2 + any other unit is illegal).

And as a timing note: if the targeted group's total is later changed above 5 (e.g., a Reaction pumps one of them), you pick a legal subset of the original targets at resolution (Rule 355.11.b; FAQ #11807).