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Only one with a maximum of two! Any excessive use of a certain technique makes it closer to misuse than use! That's a hard lesson authoritatively taught by Google (in SEO) & followed by others.
A fair practice is to use not more then three hashtags a post. If your content relates to more then three hashtags, look at it twice as you may find your content irrelevant or less focused somewhere. Keeping it within three tags or less will help you in creating posts which are "to the point".
Depends, but1 is great. You make a point and targeting on main subject. Others will be discovered easily
Depends on the content.
