How do you determine how many HD an specific monster has? I honestly haven't the slightest idea as to how you would even begin to determine this. X3
Each monster entry in the Bestiaries has its own HD. For example, if you look at the troll you'll see the second line under the Defense section lists hit points. In parenthesis after the hp total is its HD plus extra hp for its Constitution modifier (6d8+36). The troll is a 6 HD creature.
If you're creating your own monster, it depends on what you want its CR to be, and what type of creature it is.
Thank you sir, that's everything I needed to know. ^_^