If I have one Mask of Foresight and I start a showdown with only one sprite token and then react with an ambush unit, does my sprite token get the +1 from the Mask of Foresight?

Yes, your Sprite token will receive the +1 Might bonus from the Mask of Foresight.

According to the RiftJudge FAQs (specifically FAQ 2776 and 6983), the timing works as follows:

1. Initial Chain: When your Sprite token enters the battlefield alone, it gains the Attacker or Defender designation. This meets the condition for Mask of Foresight's triggered ability ("When a friendly unit attacks or defends alone").
2. Triggered Ability: The trigger is placed on the chain. Because the condition "alone" was met at the moment of designation, the Mask of Foresight trigger is locked in.
3. Resolution: You can choose to let the Mask of Foresight trigger resolve first. When it resolves, it grants +1 Might to your Sprite token.
4. Reaction: After the trigger resolves, you still have the opportunity to play your Ambush unit as a reaction. Even though you now have two units on the battlefield, the bonus from the Mask of Foresight has already been applied and does not retroactively disappear.

Summary of Timing:

• The condition for the Mask is checked only at the moment the unit is designated as an attacker or defender.
• Once the Mask of Foresight trigger is placed on the chain, its effect will resolve and boost your unit regardless of whether other units join the battlefield afterward.