If I use En Garde on a unit on Abandoned Hall, does it give +1 or +2?

Whether En Garde gives +1 or +2 might depends on whether the unit you target is the only unit you control at that location.

According to the card text of En Garde:

• It gives a friendly unit +1 might this turn.
• It grants an additional +1 might (+2 total) if that unit is the only unit you control at that location.

Because Abandoned Hall is a battlefield, you can play En Garde on a unit located there. The presence of Abandoned Hall itself does not affect the calculation—only the number of units you control at that specific location.

Summary:

• If that unit is the only unit you control at that battlefield, it receives +2 might (+1 base +1 additional).
• If you control other units at that same battlefield, it receives +1 might.

This is further supported by RiftJudge FAQ #8886, which clarifies that the bonus condition for En Garde checks if your targeted unit is alone among your units at that location, regardless of whether you are attacking or defending, or who controls the location.