one or two; a few
Used to indicate a small number, often in contexts like 'one or two things' or 'a few people'. The 'first and second' gloss is a literal reading but not a common standalone meaning.
I'll give one or two examples to explain.
I'll explain with one or two examples.
A few questions came up at the meeting.
This job will probably be finished in a day or two.
If we wait another year or two, the situation should change.
若干 is formal, written-style wording for 'a certain small amount,' whereas 一二 is neutral and common in everyday speech and news.
いくつか is a colloquial general word for 'a few (of something)' usable on its own, while 一二 typically attaches directly before a counter, as in 一二日.
少々 means 'a little' and often refers to a small amount or degree, while 一二 specifically means a small count of 'one or two' units.
four-of-a-kind and a pair
Compound of 一 (one) and 二 (two), literally 'one two', used idiomatically to mean 'a few'.