Well, I had 5 mature at retrieval, which I was very upset about and the Doc thought we'd probably do a 3 day...but 4 fertilized and all 4 are still going strong.
I don't think the research is set on which is better, he told me 3 vs 5 was an issue of being able to decrease the chance of multiples more accurately. Not sure if that's true!
Hopefully you get the best result possible!