"I need some good advice."
Translation:Eu preciso de um bom conselho.
‘ "Some good advice" is plural. ’
No, it isn't. ‘Advice’ is a mass noun to begin with and ‘some’ isn't used here in the sense of ‘an unknown number of’ but in the sense ‘in some sense not yet known, unknown or unspecified’. If you want to translate it, use ‘algum’ with the singular.
But in this sentence, ‘some’ is primarily there to contrast with ‘I need good advice.’ which would imply the speaker already has bad advice and isn't content with it.