One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.
Another difference lies in the overall mood. Canon AOT has a rather dark, gritty, and often hopeless mood due to the constant threat of Titans. However, a friendly Titan fanfic would likely have a more positive, hopeful mood. It might explore themes of redemption for the Titans, or how their existence could be beneficial to the world of AOT. This is a complete departure from the doom - and - gloom feeling that often pervades the official AOT story.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
To find the best AoT fanfiction, consider the length of the story as well. Sometimes a longer fanfic can develop more complex plots and characters. But shorter ones can also be great if they're really well - written. Check the tags on the fanfics too. If it has tags that match your interests, like 'Eren x Mikasa' or 'Titan mystery', it might be more likely to be a good read for you. And don't be afraid to sample a few different ones. You'll soon get a sense of what makes a great AoT fanfiction by reading a variety of them.
I'm not sure exactly, but there are quite a few volumes of the Attack on Titan manga. You might want to check reliable sources or the official publisher for the precise count.