Learn Russian: У нас ни копейки, а нас пятеро! - We do not have a single kopeck, and there are five of us!

When stating the size of a group using a personal pronoun (meaning 'there are five of us'), Russian requires the pronoun to be in the Genitive case (нас). Furthermore, for groups of people, collective numerals like пятеро are strongly preferred as predicates over cardinal numbers like пять.

Does the rule for нас пятеро apply to all numbers?

This construction with collective numerals is typically used for groups of two to seven people (двое, трое, четверо, пятеро, шестеро, семеро). For larger groups, you revert to standard cardinal numbers, like нас двадцать (there are twenty of us).

In the first half, why does копейки take the Genitive case, and why is there a ни without a нет?

The particle ни combined with a noun in the Genitive case expresses an absolute lack of something ('not a single...'). It acts as a stronger, emphatic version of у нас нет, so the negative word нет is simply implied and can be omitted here.

Why connect these two clauses with а rather than и?

The conjunction а sets up a contrast between the two clauses. It emphasizes the unfortunate contradiction of the situation: having absolutely zero money on one hand, while having five mouths to feed on the other. It translates smoothly to 'and yet' or 'but' in this context.