Learn Italian: Per quanto ne so, è una regola generale. - As far as I know, it is a general rule.

When per quanto introduces a limitative clause (meaning "as far as"), you can use either the indicative or the subjunctive. Per quanto ne so is very common in everyday speech. Using the subjunctive (Per quanto ne sappia) is also perfectly correct; it just sounds slightly more formal or adds a tiny bit more uncertainty.

Why is there a ne in Per quanto ne so? English just says "As far as I know."

In Italian, this fixed phrase requires a pronoun to indicate what it is you know. Here, ne translates to "about it" or "of it" (literally: "As far as I know about it"). While English leaves the topic implied, dropping ne here would sound slightly unnatural to an Italian ear.

Are there other ways to say the exact same thing?

Yes, you will frequently hear a quanto ne so or per quello che ne so. They are completely interchangeable with per quanto ne so in this context, and they all keep the pronoun ne.

Could I use conosco instead of so?

No. You must use sapere (conjugated here as so) because you are talking about knowing a fact or a piece of information. Conoscere means "to be acquainted with" a person, place, or object, which doesn't fit the phrase "as far as I know."