How much rain could a reindeer if reindeer?

This is a play on words, referencing the famous line from the song "Rudolph the Red-Nosed Reindeer".

The line is actually "If you know what I mean".

So, the answer is: Reindeer don't rain, and they don't have the ability to "rain" anything.

The phrase is meant to be funny and nonsensical, like asking "How much wood would a woodchuck chuck if a woodchuck could chuck wood?" 😉