You can register up to two emojis to send as reactions. For example: 😀😀

However, there appears to be a bug where you can register an unlimited number of certain emojis.

For example: 🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️🙂‍↕️

My guess is that the latter are emojis that can be represented by combining two emojis into one.

🙂‍↕️ = 🙂 + ↕️

🍋‍🟩 = 🍋 + 🟩