Answer:
1. Uso más de un cepillo de dientes.
It translates to I use more than one toothbrush. It does makes sense grammatically. I just don't understand why lol.
2. ¿Quieres menos que tres dormitorios en tu nueva casa?
Translates loosely to "Do you want less than 3 bedrooms in your new house?" This is not grammatically correct so you have to say. "Quieres menos que tres dormitorios en tu casa nueva?"
3. Somos más de 20 en la clase.
I don't quite understand what this is saying but it translates to "We are more than 20 in the class." This isn't grammatically correct and I'm assuming this is saying we have more than 20 in the class so you just change somos to tenemos. " Tenemos más de 20 en la clase."
4. No necesito más que un problema.
This translates to "I don't need more than one problem." It is gramatically correct and it kind of makes sense.
Hope this helps! Sorry for long answer lol!