I can't speak for the number of older teens in the whole group, but out of the 40 some cabins that are in our FE, there are 3 in that age bracket, and one 15 year old. Plus you makes 5
If you're going to be 17 I am sure you can do the math. There are 4000 cabins onboard. I just gave you a sampling of 40 cabins. Out of which there were already 5 in that group.
As for our teens experience, there were about 15 to 20 in that age group last time. Each cruise is different.
Something we have learned from our 5
Disney cruise (all when the kids were pre teens to teens), that I have shared with the group before, is that you need to go with an open mind. This will not be exactly like your last cruise. If you expect it to be, you're setting your self for disaster (quoted from our previous teen cruiser). On our second cruise, our oldest had certain expectations. When things did not go the way he expected, he ruined everyones trip. He knew better on our 3rd cruise. He went with an open mind, excited to experience new things. We let him help with the planning, including looking at available Navigators to see how he wanted to spend his day.
So, don't despair. Plan to have fun, and every thing else will fall into place.
Finally....a piece of advice from someone who's been there (specifically a past teenage Disney cruiser)
Be optimistic. The worst that could happen is that there are only 5 in your age group, and you only connect with 1 person. If you have an open mind, the 2 of you could have the time of your lives.