Do some research on the topic if you really want to know how much. [This article](http://www.growthmindsetmaths.com/uploads/2/3/7/7/23776169/mindset_and_math_science_achievement_-_nov_2013.pdf), for example, talks about how just having one of the two mindsets can dramatically alter student achievement. It also has several citations to studies specifically discussing the malleability of intelligence.
There is a large body of research that suggests that mathematical aptitude is not fixed and therefore, your position, as stated numerous times in this thread, that some people just can’t do math is just wrong. Basically, those without a learning disability who “just can’t do math” are generally just those who gave up early and just never learned how. They are mentally capable. They are also typically those who clearly believe that intelligence is fixed and cannot be improved.
I would isn’t want any teacher teaching any particular subject to pass that view onto my kids. I don’t care if it is math or English or music. I don’t know what kind of subject a given kid will grow up to enjoy, but if they are discouraged in it early, they may never get the chance. So like I’ve said, I don’t care if a music teacher can’t do calculus since it’s generally not a high school graduation requirement and isn’t all that useful for most people in their lives or jobs. On the other hand, algebra is something everyone ought to be able to do, is useful purely in a mathematical sense to most people (whether they realize it or not), and is useful just in terms of logical thinking. I do care if the people teaching children gave up on something that early.
Of course in reality, music teachers are not disqualified based on their mathematical abilities and there are definitely teachers out there who are ready to dismiss a kid as just being bad at math or any other subject. Luckily, a parent can probably teach their kids otherwise and keep them willing to work at things that are difficult but worthwhile.