Does anyone have statistics on how many people make the climb (and how many don't)?

I believe that well prepared climbers have around an 85% chance of reaching the Uhuru Peak _base_d on previous climbs. The people who don't make it, either get too exhausted or suffer from strong headaches and nausea due to the altitude. There is no telling who will make it, but the best advice is to take it slow, drink lots of water and pay attention to symptoms of altitude sickness.

Yes that is true, however even an average person, who is healthy, can increase their chance of reaching Uhuru peak is they have a day of acclimatisation at Horombo. Infact, you have an 85% - 90% change of reaching the peak if you have a day of acclimatisation.
Obviously by considering the advise given by William, which many people should heed,and yet many do not realise.
Just an idea for all those Mountain climbers , Rongai and Marangu are the easiest, while Machame, Shira
and Lemosho provide more difficult ascents.
I suggest Zara Tours in Moshi, if you ever consider climbing Kili, or you can go through Ranger Safaris _base_d in Arusha to book the climb.