They're popular enough for them to have been mass-produced, as opposed to, say, just writing the text on the cake with frosting.

That's not the reason though. 24 (vinte e quatro) is the gay number because it sounds like "vim de quatro" which means something like "I came on all 4s"

It's a very popular joke. I have no idea how many people take it seriously, since those would probably not say they do.

I imagine most people that buy a candle like that do it for somebody else's birthday to imply the other person is gay.

as a Brazillian, I have never seen the correlation of the number 24 with "gayness"

It's probably more of a joke.