LESS (noun): "Hundreds of soldiers arrive, but less [of them] remain(s)"
In general "less" is singular and "few" is plural. This is because "less" is used uncountably and "few" is used countably.
Examples
Less is more (singular)
Fewer are expected for the meeting during Covid restrictions. (plural)
Answer
Hundreds of soldiers arrive every day, but fewer remain. (plural)