I spend too much time on ET's 737-800s alas (75+ flights on this type alone and flown on every single aircraft in the fleet). The seat numbering is now standardized on all of them, but there are some other variations noted below.

Row 11 is the bulkhead with extra legroom, but with narrower seats and fixed armrests. You WILL be moved from this pre-reserved seat to accommodate a bassinet if there are any infants requesting one so I try to avoid reserving it in advance.

Row 16 is non-reclining with no extra legroom. AVOID AVOID AVOID.

Row 17 is non-reclining with extra legroom, but with fixed armrests.*

Row 18 is reclining and with extra legroom, but with fixed armrests.*

* - Except on ET-ATV which has movable armrests in rows 17 and 18 only.

On ET-APF/K/L/M/N/O only, the window seats in rows 17/18 have thinner seat cushions and are very uncomfortable as the seat metal bars dig into your back.

On ET-AQM/N/O/P/Q, the legroom in row 11 is very slightly (about 1") less than on the other aircraft (other than ET-ATV which has a different LOPA) due to the bulkhead design but still significantly more than regular seats.

On ET-ATV, the seat pitch behind the exit rows (row 19 onwards) is slightly tighter than on other aircraft due to an extra lavatory at the rear.

Hope this helps!