If I attacked with multiple units and successfully conquer but one gets stunned with back off, does the stunned unit get recalled to base?

To answer your question: No, the stunned unit is not recalled to your base.

According to the rules of combat and the established flow of the Resolution Step, the recall of attackers is conditional upon the presence of defenders.

Why the stunned unit stays:

1. Recall Condition: According to Rule 461.1.a.2, the "Combat Special Cleanup" requires: "Recall Attackers present at the Battlefield if Defenders are still present."
2. Successful Conquer: If you have successfully conquered the battlefield, it means that all defending units have been removed (either killed or otherwise moved to a non-battlefield zone).
3. No Defenders Present: Since no defending units remain at the battlefield, the condition for recalling your attacking units—including any that are currently stunned—is not met.

Key Considerations:

• Stun Status: Per Rule 423.1.b, while a stunned unit cannot contribute might to combat damage, it remains on the board and is subject to being killed if it receives damage equal to or greater than its might. Being stunned does not make a unit immune to damage, nor does it cause it to be recalled if it has survived the combat and the battlefield is no longer contested by defenders.
• Recall Timing: Recalls occur during the Combat Cleanup (Step 3 of the Resolution Step). Because you have successfully cleared all defenders, your units (including the stunned one) successfully establish control of the battlefield per Rule 461.5.

In summary: If you successfully conquer a battlefield, your attacking units remain there. Even if one of your units is stunned, as long as there are no defending units left to trigger the recall condition, that unit will remain at the battlefield and recover from its stunned status at the beginning of the next Ending Step (Rule 423.1.a.2).