What is the minimum score for the GRE subject exam in mathematics? Unfortunately, the admission website of a university I am interested in applying to does not list the minimum score required on the GRE math subject exam. 

Does anyone know the minimum score requirement for this exam when applying to graduate schools to study mathematics?
There is no definite minimum GRE score (other than the smallest possible score on the test) for any program, unless that program explicitly has adopted one. If they don't mention it on their website, you could try calling or emailing their graduate coordinator and asking them. 

In all, here are the main possibilities, and the reality varies by institution:

Case A) There is no minimum, and they consider all applications. 

Case B) There is a minimum, and they will tell you.

Case C) There is a minimum, but it is a secret.

Case D) There is no formal minimum, but there is a de facto minimum in that they almost never (or never) actually accept someone below a score of X, even though they technically say there is no minimum. In the end, this is the same as Case C above.

In practice, the most selective institutions will occasionally mention "averages" like 80% percentile or 90% percentile - but without a standard deviation that's not really very informative. It is also the case that many people view GRE scores differently depending on your country of origin, so it is not necessarily meaningful or helpful to know anything like averages.

In the end, the advice for the GRE is: do as well as you can without harming some other aspects of your application (don't let your grades slide just to study for the GRE), and as always try to have the best overall application packet to maximize your chances.