複数 and 多数: more than one and an assessment of many
Separate the condition of more than one in 複数 from the assessment of many in 多数, without inferring a large number or a majority from plurality.
More than a single item
複数, read ふくすう, indicates more than one. 複数の案を比べて一つを選んだ。 says that several proposals were compared and one selected.
There were multiple proposals for comparison, but the final selection was one. Do not transfer 複数 to the number selected.
Expressing a large number
多数, read たすう, expresses a large number. 多数の作品が展示されています。 says that many works are on display.
複数の作品 can apply to two works; an assessment of many depends on the context or comparison base. Do not define 多数 as exceeding half in every sentence.
Possible answers and actual selection
同じ質問に複数の答えが考えられる。 says that multiple answers can be considered for the same question. It does not report submitting all of them.
候補は複数ありますが、選ぶのは一つです。 says that there are multiple candidates but only one is to be selected. Keep candidate count separate from selection count.
How many proposals were finally selected in 複数の案を比べて一つを選んだ。?
Show the answer and why
One. 複数 refers to the proposals compared, while 一つ specifies the selection result.