Help understanding ESD ratings
What is the normal and maximum human body static voltages and what cause it to increase?

There really isn't a normal maximum, its dependent on the triboelectric charging between the materials your wearing (the source of the charge) and any discharge path (humidity in the air also makes a difference). See this article 

What does the value 500V mean? is it safe to touch it with hand for
example?

That means it was tested to 500V, which mainly concerts input terminals of the device that would be connected to the 'outside' world by a cable or something the user of a device could touch. The human body model is: &quot;this standard is intended to simulate a person becoming charged and discharging from a bare finger to
ground through the circuit under test.&quot; it involves a set capacitance and resistance and a standardized test setup.
If you want electronics to survive then get a proper ESD setup. There are numerous articles on this site and the internet that describe what is acceptable for hobbyists. An ESD mat and a wriststrap would be a good start.

How can I tell what's my ESD voltage at a given time?

You could build or buy an electrometer and measure your potential between you and ground.