"What is the maximum a security guard can make in HK?"
While it is likely that the vast majority make at or near minimum wage, your question is somewhat in line with "how long is a piece of string?". The maximum a security guard can make depends on how much an employer is willing to pay them. At my building I know that our guards are paid a fair bit more than minimum wage, but they all also speak excellent English, which is something that the tenants asked the management for (which also increased our monthly management fees proportionately to the increased salary for guards with an extra skill set). However, this is rare in HK and pecular to only a few small estates that I know of.
Salary can also depend on whether the guard has any supervisory role. For example, at Parkview, the guards have a variety of ranks with various levels of supervisory duties and corresponding different salaries.