I think you need at least 500 bees to survive winter and insulation.
20 000 bees occupy a langstroth box. When it makes a winter cluster, it has bees only in 4-5 frames. That is a minimum wintering hive.
500 bees is nothing.
But no one count hive size as number of bees. They are counted as frames.
5-frame nuc is practical size of wintering nucs, because hive should have enough bees to start spring build up.
Before spring the hive looses lots of bees and no one can say what they have in spring and how it starts build up.
The winter cluster will be same size as brood area in late summer before winter beeding.
If you have 5 frames brood, you should not use the whole box for wintering. You need a dummy board.