For tax purposes, custody is based on the number of nights that the child 'belongs' with the parent.  They need to keep track of it very carefully  So except for leap year (such as 2020), there is no such thing as 50/50. 

However, it might be a good idea for the parent whose 'year' it is, to ensure that the kid stays with that parent more nights.  That way, YES, the parent can potentially claim all of the other tax benefits.