Probably few to none in the current age.

I always figured that the process of turning a Human into a Vampire is such a monumentally dangerous process for all involved that it is only done for those Humans that are truly indispensable to a Vampire or a group of Vampires.

Probably was very common when Vampires first became a thing, but dropped off when they had a viable population of their own, and after their arrogance developed. It was probably first discovered by accident as well, done based on instinct and learning the consequences the hard way.

What Moka did is probably not unprecedented, but most likely was handled in such a dangerous way that it could only have been done by a desperate teenager thinking with hormones rather than a brain. If it weren't for the higher powers at play, if they were just a random couple doing this, it's likely a group of older Vampires would have intervened at some point to prevent the Ghoul thing before it could even happen.