"I made it rain" can mean anything that you throw in the air and then it falls back down over a surface as though it were raining. It is often used as slang term for when you throw a great deal of money in the air and it has the effect that it is raining money. You will find the term used a great deal in hip hop culture and it is usually something that you say if you are in a nightclub, or more specifically an adult entertainment establishment, and you throw money up in the air so that it rains down over an exotic dancer. Do you have the full sentence where the term was used so that I can tell you exactly how it was meant?

Sheer numbers = very large numbers eg Obama won by a sheer number of votes I made it rain, may mean the person did a rain dance and by coincidence it started to rain.

I don't know any meaning of "I made it rain" other than the literal words... I suppose if you were in a plane seeding the clouds with silver iodide to make it rain... (does anyone still do that?) sheer literally means clear or transparent, like nylon stockings may be sheer. but figuratively sheer means 'simply that, nothing else'. When the monster appeared we were gripped by sheer terror. Since the teams were 20 against 3 it was clear the game was won by sheer numbers alone.

"I made it rain" <-- meaning in context could be slang to waste money on things like prostitutes or strippers to make it "rain money"

[sheer] in this context, sheer does NOT mean clear or transparent. here it means, completely/totally/purely by **something** alone example: By sheer numbers, the barbarians won against the knights. so, this means that the barbarians won because of number alone (even though they have no training, armor or weapons)