From Relequen's description, you generate a stack by a light attack. It lasts 5 seconds. So absent an attack speed buff, it would seem that there never could be more than 5 stacks.
Yet if that were true, it would be a terrible set. Further, the 20 stacks limit wouldn't need to be in the description. What mechanic am I overlooking?
And are the particulars different in important ways for other similar sets? E.g. Siroria?