Maths exists nowhere but in working brains (and not necessarily in all of them) because concepts are only found in working brains. The entities of maths are all abstractions, and abstractions are a form of concept with no real counterpart (eg the difference between 'this chair', real counterpart possible, and 'a chair', no real counterpart).
That's why you never find uninstantiated twos running round naked in the wild. Or real uninstantiated π's or
e's or i's and so on.
(And you never find real points, real lines, real planes, either. They too only exist as concepts with no real counterpart.)
Not only that, but before you can count to (an instantiated) two, you must first make two choices: what things you're going to count, and the field you're going to count them in. How many
sheep in the
barn? How many
animals in the
barn? How many kinds of grass / blades of grass / seeds of grass in this nook / lawn / golf course &c. The choices don't exist in nature ─ the onlooker has to make them.
Plato's theories of 'forms' says that each instantiation eg 'this chair', gets its quality of being a chair ─ 'chairness' ─ from the form 'perfect chair', which has, if not objective existence in the sense of being found in nature, some kind of existence independent of the onlooker with some kind of access to the chair in question.
And the idea that maths exists independently of the concepts of maths found in brains is called 'mathematical Platonism'.
Since in preferring the 'concept' concept, I reject Plato's 'forms', for the same reason I disagree with the mathematical Platonists too, including your Max Tegmark, and Roger Penrose, and not a few others.