Can you ask for a referral to a dietician? They would be able to help you figure out what is the correct amount.
Also, if you're unsure what your mum means when she says things like that, maybe you could ask her to explain?
I think there's definitely grey area between healthy and unhealthy personally. You can be eating enough calorie wise but not getting the correct amounts of vitamins and minerals you need for example. Or you can not be eating enough while eating super clean and "healthy", and that's still not going to change the fact that you're restricting and not getting all the right vitamins and minerals either. So I don't think it's super black and white. But also what is healthy for one human isn't going to be the same for another. We all have different bodies and different needs.