1) Do you read fanfics that have nothing to do wih the mane 6 or background characters?
Not unless it's recommended. I'm mostly interested in the ponies I know.
2) Do you also read fanfics that are focused around oc's?
Same as the first question.
3) What do you think makes a good fanfic?
A clear idea, in-character or believable out-of-character ponies, and well-written as far as grammar, spelling, and punctuation goes. Fanfiction writers typically don't have the experience of or put in the same amount of hours, planning, and editing as known names in literature so I don't hold them to those standards, but it's a different thing entirely to not fix run-on sentences or correct minor errors. It just seems sloppy and like they're not proud of their own work if they leave it like that, because most fanfiction could probably be spruced up in an afternoon with a little assistance.
4) Are humans common in fanfics?
They seem pretty common to me, but I'm not surprised. It's always been a popular choice to write about someone from our world ending up in the fictional world and meeting the characters.
5) What is the best fanfic you have read?
Can't pick right now.
6) Do you like a story better if it has humans in it or if it is only ponies?
I prefer ponies but I'm open to stories with humans.
7) How many stories have you read?
I only finished 7 in this fandom so far.
8) What genre of fanfic do you like to read?
Drama, adventure, and sci-fi/fantasy are my favorite genres in general so I gravitate towards those, but I also like stories that reflect the show and are about friendship and character. I especially like things that shed light on a fictional universe or fictional character that canon left unexplained or untouched.