If you are making a game just for yourself, the risk of legal action is low, though still present. However, if you intend to sell your game on Steam, Itch.io, or other platforms, using an illegitimate RPG Maker MV product key is professional suicide. Publishing platforms require you to own the rights to every asset and tool used in your game. If Kadokawa (the parent company) discovers you are selling a game made with a pirated engine, they can issue a DMCA takedown. This can get your game removed, your developer account banned, and your reputation destroyed before you’ve even started.
Searching for a free "RPG Maker MV product key generator" is a massive risk. Here is why: