The US Bill of Rights guarantees the right to a jury trial in all civil suits exceeding 20 dollars. Did twenty dollars have any particular significance in 18th century civil law, and did the framers intended for that amount to change with inflation?

The history of the Seventh Amendment--in its entirety, not just the $20 clause--is rooted in English Common Law. And actually, the Supreme Court has extended the guarantees of the amendment many times through applying a "historical test." This meant that because the amendment was based on English Common Law, that that was the relevant guide to what the Founders intended.

So was there something special about $20 in English common law? It's possible, but considering the fact that ECL has never been formally codified, it seems unlikely to me.

Another option might be that they just thought it was a good baseline, but I'm a little skeptical of this too. Twenty dollars in 1790 was worth approximately $500 in today's money(You can look this up using an inflation calculator). That's a fairly substantial amount but still pretty small. Considering how much more localized everything was back then, maybe the idea that a federal case involving such a paltry sum wasn't crazy. It would've cast a larger net for sure, perhaps guaranteeing a trial by jury for a greater percentage of interstate or international disputes. So that could have been the intent behind it.

But regardless, I don't think we can confidently say that the framers really thought all that much about the amount or the effects of inflation. They definitely didn't make it easy to amend the constitution to make such adjustments.

It's generally agreed that the $20 clause has little to no bearing on the principle of the Seventh Amendment, which is the right to a trial by jury in civil suits. The clause has been disregarded as obsolete and ignored by federal jurisprudence and even by current federal law. Also it's worth noting that the Seventh hasn't been incorporated against the states, so the clause has no bearing in state courts either.

Random aside: It would definitely be an interesting case if someone decided to challenge the federal courts on the grounds of violating the $20 clause. Most likely, you'd need something really weird to happen in DC where all civil suits are tried in federal courts because the city isn't a state.