8 Hatched... 15 more eggs and no pips??

"Normal" when it comes to hatching eggs covers quite a large area and I don't think there is an "average" to be honest. There are so many factors, things like the breeders' age, their diet, the age of the eggs, handing of the eggs prior to incubation, incubator settings and cleanliness, management of the incubator, that all influence the success of the hatch to some extend. I've been hatching eggs for a few years, under broody hens and in incubators, from rubbish home-made incubators to my current Brinsea and one thing I learned is that hatches are unpredictable. I've seen a chick under a broody hatch on day 25, two full days after the last of her siblings hatched. I've had early hatches, I've had late hatches, I've had eggs I thought were off hatch and I've seen chicks go full term and die on me, I've had 100% hatches and I've had 5% hatches. If you did your best, with fresh eggs and correct incubator settings, all you can do is keep your fingers crossed and see what happens. Leave those eggs another day or two. If they don't make it, let us know what you did, what was your temp and humidity settings, etc and we can help you figure it out. Good luck!