I am curious as to what is the guideline or are the guidelines for the use of a plural version of a noun that could be either countable or uncountable. Speech after speech, writing after writing, I think I have seen and heard the use of a plural in cases where either option seems to be viable. Merely thinking that should be used in the mind of a writer is a sufficient ground for such a use?

... he points us beyond styles and termperaments to recongnize ...

The writer didn't seem to have given us a clue as to why a plural version is necessary but went on to use the words 'styles' and 'temperaments' instead of 'termperament' and 'style', which I think could have done the job just as well, considering the lack of prior clues as to their coming in use.

So, a mere fact that a person desires to use it is a good enough reason to use it or them?
Anonymousa plural version of a noun that could be either countable or uncountable.
There's no such thing as an uncountable noun in the plural. Plurality is one of the marks of countability. Once the noun is in the plural you know you are dealing with a countable noun.
Anonymousthat a person desires to use it is a good enough reason to use it or them?
Not just desire. Very accomplished writers with a fine ear for the language are able to gauge when it is useful to present an apparent uncountable in the plural -- especially when it's a matter of abstract nouns. The majority of us merely competent writers don't typically invent such novel turns of phrase.
The writer must be referring to various styles and temperaments in his or her writing.
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Thank you. What should I use for this?

Possible CEO saying: We have a crisis and have to pull ourselves out of this financial rut we are in. OK, guys, what do we have to do?
A brave young man: I think, sir, we have to hold various discussion (or various discussions??) on the matter online due to the distances we have to cover.
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Thank you. I think I was not trying to invent any fancy novel turns of phrase as I don't think I am at such a stage yet, but I was trying to do was gain a grasp of basic guidelines (if those are avaiiable) for me to make sound decisions in situations like below had I been unfortunate to be in one.

1.We had many discussions on how to recoup our sagging financial performance levels yesterday.
OK, we had much discussion (or many/a lot of discussions??) on our sagging bottom lines and I think we need to find some solutions if that is possible as soon as possible? Do you have any good ideas?

2. Joe flaundered and did all the bad things, and spent all the money he inherited.
Father: You became what you are because of your past sin (or sins??).
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Anonymousbasic guidelines (if those are avaiiable)
Oh. I see what you mean better now. Unfortunately, I don't know of any guidelines like that. Maybe other members of the forum can help you on that.

As for your specific examples,
We had many discussions ... (countable plural) seems best to my ear.

We had a lot of discussion on ... or There was a lot of discussion on ... (uncountable) are also possible.
In the second example, I sense that only a countable version of sin is possible because specific examples were given and a possessive adjective is used.

Multiple sins were mentioned, so you need sins (countable):
... because of your past sins.
If only one sin had been mentioned, you would have needed sin (countable):
... because of your past sin.
If you wanted a generalized idea of sin (uncountable), you would need a different phrasing:
... because of sin.