I'm taking into account how many have driven each car as of the end of S12 and have posted a time on either the celeb board or the F1 board. To date, 71 have driven the Liana (yes, 65 if you don't count the F1 drivers or Stig), 46* in the Lacetti. Presuming there is one SIARPC every episode in the next two series, and none of those guests are F1 drivers, and also presuming there isn't a road trip special (a la the US fly-drive (9x03), Botswana or Vietnam) which would preempt the segment entirely, the total number of drivers for the Lacetti, after Series 14, will be 60--just five shy of the Liana's final total, discounting the F1 drivers and Stig. (Taking F1+Stig into account, the total would be eleven shy, of course.)
* I miscounted the total in the Lacetti for S8. It's fourteen (seven in 8x01 to get the new board started; the other seven in each of the following episodes, obviously not counting Jenson Button (F1 board, Liana)), not fifteen.