"Žofie was a mother of ten children."

Translation: Žofie byla matka deseti dětí.

[ion1122]

English would amost always use the definite article "the" here, even when no specific mother is being singled out.

So, the English we are asked to translate should be "Zofie was the mother of ten children", and we should translate it into Czech without any article, as above.

[mattvick]

In English we could also write “Zofie was mother to ten children”. Which seems grammatically closer to the Czech but actually means that Zofie acted as a mother to ten children, but did not necessarily give birth to those children. How would that be expressed in Czech?

[VladaFu]

I changed the default to "the mother".

[emjee5]

I think "byla matkou" is the correct expression

[VladaFu]

Both cases, nominative and instrumental, are perfectly fine here.