(Correct answer to the question is 9. But, the prequel is garbage and is something I regret posting. The Void spin-off was not meant to be legit apart of the story but has influenced V2 so it is "canon" now. I'm going to add the original Asterisk tag on this post for anyone wanting to read everything previously, if it will even work. ANW and the prequel can be skipped since those two don't really do much story wise except build character. (Don't read the prequel).
Seasons 1 and 2 of the original story focus on the dungeon just like V2. Seasons 3 and 4 are on earth in case you want to skip around. ANW still on earth. The Void is mainly fighting and wasn't finished.
Note: confusion might occur. V2 was meant to be fresh from the confusion of previous seasons but, I can answer stuff if asked directly.
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V2 feels really fresh to me, and I think it will stay that way as long as you don’t introduce any more of the old characters. [spoiler]buuuuuump[/spoiler]