Here in South
Africa, I've only seen it with a double m: Emmerentia. There is a suburb in Johannesburg called Emmerentia, with a dam where people go rowing.
I find it rather a mouthful for everyday use: I prefer four-syllable names to have the main stress on the second syllable, like
Victoria or
Felicity.
That said, I know a woman named Rentia (pronounced Rensha), which I like very much. She is Rentia
Elizabeth.