If I play Brawler Sett, he gets a +1 buff and then 2 turns later I move him to an empty battlefield, does he get a +2 from the buffs or do they not stack?

In Riftbound, buffs do not stack on a single unit.

According to Rule 702.3: "There can only be one Buff on a Unit at a time." Furthermore, Rule 702.3.a specifies: "If a Buff is added, or instructed to be added, on a Unit that already has a Buff, it is not placed instead."

For Sett, Brawler, his ability specifically includes a reminder of this rule: "When I'm played and when I conquer, buff me. (If I don't have a buff, I get a +1 might buff.)"

The Scenario:

1. Played: When you play Sett, he gains a +1 Might buff. His total Might becomes 5.
2. Moving/Conquering: Two turns later, when you move him to an empty battlefield and he conquers it, his ability triggers again.
3. Resolution: Because he already has a buff from when he was played, the new buff from conquering is not added. He remains at +1 Might (Total 5).

To get the most value out of Sett, you should spend his current buff (activating his ability to give him +4 Might for the turn) before the conquer trigger resolves. This way, he is no longer buffed when the "When I conquer" ability resolves, allowing him to gain a fresh +1 buff.