With Master Yi Legend, do you have to declare the plus +2 in combat at the start of combat or is it default without any declaration?

You do not need to declare the +2 might bonus from Master Yi (Wuju Bladesman). It is a passive ability, not a triggered ability, and it applies automatically by default whenever its condition is met.

According to RiftJudge FAQ #8089 and #9025:

• Passive, Not Triggered: Master Yi's ability is a passive ability that is constantly checking the game state. It does not use the chain and does not require you to announce it to "activate" it.
• Automatic Application: As soon as a friendly unit gains the "Defender" designation and is the only unit defending at that battlefield, the +2 might bonus applies immediately.
• Etiquette: While you are not required to proactively announce it, you cannot lie about it if your opponent asks. It is the opponent's responsibility to account for the bonus when making their decisions.

In short, the bonus is always "on" as long as the condition of defending alone is met.