The Solo Leveling comic is indeed finished. It had a definite ending that concluded the various plot elements and character developments. Many readers are still discussing and cherishing it even after the finish.
There are several options. Some people find it on dedicated comic websites. Another possibility is to check out apps specifically designed for comic reading. But make sure to use legal and authorized sources.
The number of Solo Leveling comic books can vary depending on editions and reprints. It's best to refer to the publisher's information or dedicated comic book catalogs to get the exact number.
I wouldn't recommend looking for free unauthorized sources as it's illegal and unfair to the creators. You can check out legal platforms that might offer free trials or discounted subscriptions.
I'm not sure where you can find it for free legally. It's best to support the authors by using authorized platforms that might require a subscription or purchase.
They fill in the gaps. There are many parts in the main story that left us wondering about certain characters or events. The side stories answer those questions, making the overall narrative more complete.