6 month reentry visa

Hi, 6 month multiple means the visa is valid for 6 months, it is not the duration of stay issued. For example, if you get a 6 month multiple entry X with 30 days as duration of stay, then you have to leave every 30 days and reenter to activate next 30 days.
If your purpose is for learning Chinese, you can apply for X2 type for a stay less than 180 days and medical check is not required. A single entry may be enough because it can be issued with a long duration, e.g. 180 days.