You are talking about two different 'databases' of questions: valid and verified questions and test pilot questions. (test pilot questions are not part of the validated database of questions yet.)
For NCLEX RN, there are about 10,000 actual validated questions in that database. For NCLEX PN, there are about 8,500.
So, for the RN exam, IF you get 265 questions, your odds of getting any particular question is about 1:39 (You will answer questions from about 1/39th of the database.) IF you get 75 questions, your odds drop to 1:133 (You will answer questions from about 1/133rd of the database.)
On the NCLEX RN exam, you WILL get exactly 15 test pilot questions, and they will ALWAYS be in the first 75 questions. If you are on question 76 or higher, EVERY question at that point comes from the valid question database.
For NCLEX PN, you will get exactly 25 test pilot questions, and they will ALWAYS be in the first 85 questions. If you are on question 86 or higher, EVERY question at that point comes from the valid question database.
I believe you are asking, how many questions are being test piloted at any given time. I don't know the answer, but I can guess. Nursing changes all the time. Many things I did 13 yrs ago are out of date today. New therapies and technology must be addressed on a continual basis.
Another factor to consider is HOW LONG they pilot a question before they consider it validated. I don't know that answer.
Another factor is keeping questions 'fresh'. If they keep a static 10,000, it becomes easier and easier for companies to 'teach the test'. So, the same concepts would need to be represented in a different form to keep the test 'fresh'.
So, out of the 10,000 questions in the database, there is always a percentage that becomes outdated and needs to be replaced, or need to be replaced in order to be 'fresh'. What percentage? If you say that 10% of the questions in the database are being changed over any given year, then your answer would be that there are about 1,000 test questions being 'piloted' at any given time.
IF I had to guess, based on the length of time needed to validate a question, the number of questions that must be replaced because they are outdated, and the number of questions being 'retooled' in the validated pool at any given time, I'd guess: 750 - 1,500 questions are being 'piloted' at any given time.
That puts the odds of getting any given pilot question at about 1:50 to 1:100, or roughly, the same odds of getting a question from the validated pool, depending on how many questions you answer. So, I think this is probably a good guess range.
But, It''s just a guess.
(If you assume a range for pilot questions that mimics the range of valid questions as a percentage, that would bring my guess range to 585 - 1995 questions, or the odds of getting any given pilot question as 1:39 to 1:133. I think it's a safe bet that the actual number of pilot questions DOES fall in at least this range.)
Finally, the number of actual pilot questions are probably a 'float' at any given time. There are probably times when they need to replace MORE outdated questions, and times when they need to replace LESS. I imagine they check to see how many of their questions are compromised, to their knowledge, and that would affect how many questions they need to retool at any time. So, the actual number of pilot questions at any given time is probably variable and probably DOES fall within a range and is not an exact and consistent number.
I hope that makes sense.