I have a Lillia fae fawn on a battlefield, I have a sprite and a mask of foresight in my base. My opponent move a unit to attack my Lillia, I play smoke and mirrors from hidden to swap the spot of Lillia and sprite. Smoke and mirrors resolve, sprite move to the battlefield while Lillia move to base trigger her spawn a sprite ability in her previous location, the mask of foresight trigger go on chain to give +1 might to the sprite that is currently alone on the battlefield then I chain this with en Garde to give +2 might to the same sprite who is currently alone on the battlefield. Once every resolve, I will have a 6 might sprite and a 3 might sprite on the battlefield

Yes — that line is correct, and the math checks out: the swapped-in Sprite ends at 6 Might (3 base +2 from En Garde +1 from Mask) with a fresh 3-Might Sprite beside it.

The key is your chain order. En Garde must sit on top so it resolves first: at that moment Lillia has already left and her token isn't out yet, so the Sprite is the only unit you control there and En Garde's "only unit you control there" clause gives the full +2 (not just +1). Mask's +1 then resolves and is locked in regardless of what arrives later, and Lillia's move trigger plays the 3/3 Temporary Sprite last — which notably does not re-trigger Mask, since it doesn't enter alone. This is the exact sequence confirmed in FAQ #12488 (rule refs 359.3, 385).

One thing to avoid: if you let Lillia's token resolve before En Garde, the Sprite is no longer alone, En Garde only gives +1, and you'd end at a 5-might Sprite instead of 6.