Or perhaps a turbolift shaft that runs between decks 7 and 8...
In TOS, the same corridor sets were used for portraying both saucer and secondary hull facilities. In TMP, the secondary hull decks were supposed to be higher than the saucer ones - which might well mean the loss of three decks.
Mere window row counts don't tell the whole truth about TOS or TMP, but the 20-deck ballpark is obvious from the latter, and we're supposed to think this ship is the rough same size as the former. Both the TMP refit and the E-A are externally identical, so it's difficult to see something as major as deck count being different there, even if the two ships were refitted or built somewhat differently.
We know what Andrew Probert intended for the TMP ship and her 21 decks, so we can pretty confidently force the other two ship versions into the same mold. The saucer would then have 11 decks (from 1 to 11, or from A to K), the secondary hull would have eight (from 14 to 21, or N to U), and the neck five (from 9 to 13 or I to M).
For the TOS ship, fudge a few more decks (of reduced height) into the secondary hull and/or the neck, but keep the saucer like above: its shape doesn't change all that much, and e.g. in "Errand of Mercy" we hear that Klingon weapons slam onto Deck 11 while only lower saucer hits are shown...
Timo Saloniemi