Of the 18 eggs retrieved, probably all were not mature. Of successfully fertilized eggs, one clinic said 40-50% blast rate is typical.
They didn't say the 40-50% were necessarily well-graded blasts, just that they become "blasts". Some clinics don't freeze any blasts below BB grade (AKA they don't freeze ones with a C anywhere in the grading). It sounds like you have a good number frozen on day 5, and day 5's have a higher average implantation rate than day 6 blasts.
It all sounds very hopeful, good luck with the 2ww.