If a narrator is weak in his righteousness (عدالته) the hadith cannot be strengthened, but if the weakness is in a narrator's accurateness (ضبطه) the narration/hadith can be strengthened but under certain terms. What are these terms? How many are they?
This is a very subjective and case-based issue. There are two kinds of weak ahadith: da'if (weak) and da'if jiddan (very weak). A da'if hadith is when there is some kind of anomaly with the (chain of the) hadith, e.g. one link in the first generation is missing. This anomaly cannot be overlooked, and yet it is not fatal. A da'if jiddan hadith on the other hand is much weaker and has serious and potentially fatal flaws.
The vast majority of hadith scholars say that da'if jiddan ahadith should never be used for anything, and they do not strengthen anything.
Da'if hadith on the other hand, can sometimes strengthen each other. It is very case-based, it depends on the overlap between the content of the hadith, the actual flaws that each contains (e.g. is it the same flaw or different kinds of flaws), the overlap in the chain of narration, and other things like that. Depending on the judgment of a scholar, sometimes two weak ahadith can strengthen each other, and sometimes they don't. It also depends on the topic of the hadith - is it relating to virtues of some act? Is it establishing something entirely new in shari'ah?
In short, it's very subjective and case-dependent, and many scholars come to different judgments in different cases.