In the past, the number is about 2/3 in that roughly 2/3 of candidates who end up being triple Q'ed get noms. However, that number comes in hindsight. Here's what I mean.
First, Pres noms are non-competitive. If you are eligible for one, you get one. Thus, you can get a Pres nom without being 1 Q'ed, 2 Q'ed, or 3 Q'ed. You can get one before you complete your packet. So, there can be many Pres nominees who are not 3Q'ed.
Second, please understand that, at the time the MOCs are selecting candidates for noms, many candidates haven't completed their packets. Thus, the MOCs may not know whether their choices will end up beign triple Q'ed. Some (with LOAs or otherwise) may be known to be scholastically qualified, but may not have taken their CFA and/or their medical may not be complete or resolved.
Finally, some candidates who don't get noms may drop out of the process. IOW, they may have completed 50% of their USNA packet at the time nom decisions are made by the MOCs. If they don't get a nom, they may lose interest and not complete the packet -- and they may have been triple Q'ed had they finished everything.
Thus, at the end of the process, about 2/3 of those who complete their applicatons and are triple Q'ed receive noms. However, there are those who get noms who aren't triple Q'ed, etc. It's not an exact comparison, so to speak.