When we offer advice it is almost always 'some advice' not 'an advice' so I don't see how anyone could say we never use it in the plural.
"Advice" is uncountable, so it cannot be used in the plural form. We offer some advice or a piece of advice.
I disagree. I think it is mostly used as plural. A pair of pants is one object, right? Yet we call it a pair, and we say "them" instead of "it". It's based on how you say it and should not be taken so literally. You say "some advice", you can't say "Let me offer you one advice", and saying "one piece" implies that there's more.
I dont think i have ever seen a pear of pants not used in the plural sense