Ellas escriben muchas cartas los fines de semana.

So you're correct now in how to use muchos/muchas. 🎯

- Fin is masculine.

- And in Spanish you would drop the "en".

I'm not a language teacher so I'd have to really think hard to find the logic for those two points.

If you wanted to be fancy you could say... "durante los fines de semana" but that would read more like "over the course of the weekends" rather than "on the weekends". But it would be valid.