Exactly 12 out of the 79 questions on the "classic" GMAT (15.2%) are experimentals. Despite GMAC's reluctance to disclose the true number of "pretest" / experimental / unscored questions, we now know this for a fact.
GMAT ESR (Enhanced Score Report) results allow us to infer that exactly 9 out of 12 Integrated Reasoning questions will count toward your score (the other 3 will be experimental), because
percentiles on the IR section always align with a denominator of 9.
In addition, ESR data tells us that 3 out of the 31 Quant questions are experimental / "pretest" questions, and that 6 of the 36 Verbal questions are experimentals.
Again, we are able to infer this information using the "denominator method": because Quant percentiles for each quarter always align with a denominator of 7, we know that there are 4 x 7 = 28 scored questions and 3 unscored questions on Quant.
Likewise, because Verbal percentiles either align with a denominator of 8 (first and last quarters) or 7 (2nd and 3rd quarters), we know that there are 8 + 7 + 7 + 8 = 30 counted questions and 6 unscored questions.
UPDATE 4/5/18: On the updated, shorter GMAT, the number of experimentals has been reduced from 9 to 3 on Quant, and from 11 to 6 on Verbal, beginning 4/16/18. However, the number of experimentals on the IR section (3) has stayed exactly the same. Overall, the percentage of experimental has been reduced from 25.6% to 15.2%, which is a welcome change!
GMAC has shortened the GMAT by 23 minutes as a result of reducing only the number experimental questions from 23 to 12 (6 experimental removed from Quant and 5 from Verbal). By doing so, GMAC has created an easier, fairer test!-Brian